Add an ExportToXlsx option into the native RadGridView exporting.  It has been done for WPF (http://docs.telerik.com/devtools/wpf/controls/radgridview/export/export-xlsx) - why not WinForms.  This would avoid the pain of having to implement the export using RadSpreadProcessing.

To reproduce:
Open the attached project, select two rows and delete them.

Workaround:
private void RadGridView1_RowValidating(object sender, Telerik.WinControls.UI.RowValidatingEventArgs e)
{
    if (e.RowIndex != -1)
    {
       
    }
}

To reproduce: populate RadGridView with data and use the following code snippet:
Me.RadGridView1.TableElement.TableHeaderHeight = 60
RadGridView1.PrintPreview()

Workaround:

Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
    Me.RadGridView1.TableElement.TableHeaderHeight = 60
    Dim printStyle As GridPrintStyle = New GridPrintStyle(RadGridView1)
    Dim renderer As CustomTableViewDefinitionPrintRenderer = New CustomTableViewDefinitionPrintRenderer(RadGridView1)
    printStyle.PrintRenderer = renderer
    RadGridView1.PrintStyle = printStyle
    RadGridView1.PrintPreview()
End Sub

Public Class CustomTableViewDefinitionPrintRenderer
Inherits TableViewDefinitionPrintRenderer

    Public Sub New(grid As RadGridView)
        MyBase.New(grid)

    End Sub

    Private Function GetRowLayout(row As GridViewRowInfo, fitWidthMode As PrintFitWidthMode, _
                                  hierarchyIndent As Integer, drawArea As Rectangle) As TableViewRowLayout
        Dim hashKey As Integer = row.ViewTemplate.GetHashCode() + row.HierarchyLevel
        Dim fi As FieldInfo = GetType(TableViewDefinitionPrintRenderer).GetField("rowLayouts", BindingFlags.NonPublic Or BindingFlags.Instance)
        Dim rowLayouts As Dictionary(Of Integer, TableViewRowLayout) = fi.GetValue(Me)
        If rowLayouts.ContainsKey(hashKey) Then
            Return rowLayouts(hashKey)
        End If

        Dim table As GridTableElement = TryCast(row.ViewTemplate.ViewDefinition.CreateViewUIElement(row.ViewInfo), GridTableElement)
        table.Initialize(Me.GridView.GridViewElement, row.ViewInfo)
        table.RowHeight = Me.GridView.TableElement.RowHeight
        table.TableHeaderHeight = Me.GridView.TableElement.TableHeaderHeight
        Me.GridView.ElementTree.ApplyThemeToElement(table)

        Dim rowLayout As New TableViewRowLayout()
        rowLayout.Context = GridLayoutContext.Printer
        rowLayout.Initialize(table)

        Dim systemWidth As Integer = 0

        For Each c As GridViewColumn In rowLayout.RenderColumns
            If TypeOf c Is GridViewRowHeaderColumn OrElse TypeOf c Is GridViewIndentColumn Then
                systemWidth += rowLayout.GetColumnWidth(c)
            End If
        Next

        Me.GridView.BeginUpdate()

        Dim mode As GridViewAutoSizeColumnsMode = rowLayout.ViewTemplate.AutoSizeColumnsMode

        If fitWidthMode = PrintFitWidthMode.FitPageWidth Then
            Dim state As ColumnsState = Me.SaveColumnsState(rowLayout.ViewTemplate)
            rowLayout.ViewTemplate.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill
            Dim groupLevel As Integer = If(row.Group IsNot Nothing, row.Group.Level + 1, 0)
            rowLayout.MeasureRow(New SizeF(drawArea.Width + systemWidth - ((row.HierarchyLevel - groupLevel) * hierarchyIndent), drawArea.Height))
            Me.RestoreColumnsState(state)
        Else
            rowLayout.ViewTemplate.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.None
            rowLayout.MeasureRow(New SizeF(Me.GridView.Width, Me.GridView.Height))
        End If

        rowLayout.ViewTemplate.AutoSizeColumnsMode = mode

        Me.GridView.EndUpdate(False)

        rowLayouts.Add(hashKey, rowLayout)

        Return rowLayout
    End Function

    Public Overrides Sub DrawPage(traverser As PrintGridTraverser, drawArea As Rectangle, _
                                  graphics As Graphics, settings As GridPrintSettings, pageNumber As Integer)
        Dim fi As FieldInfo = GetType(TableViewDefinitionPrintRenderer).GetField("currentPage", BindingFlags.Instance Or BindingFlags.NonPublic)
        Dim currentPage As Integer = fi.GetValue(Me)
        Dim skipPage As Boolean = currentPage <> pageNumber
        Dim drawAreaHeight As Integer = drawArea.Height
        Dim currentX As Integer = drawArea.X
        Dim currentY As Integer = drawArea.Y

        Dim rowLayout As TableViewRowLayout = Me.GetRowLayout(Me.GridView.MasterView.TableHeaderRow, _
                                                              settings.FitWidthMode, settings.HierarchyIndent, drawArea)
        rowLayout.IgnoreColumnVisibility = settings.PrintHiddenColumns

        Dim systemWidth As Integer = 0

        For Each c As GridViewColumn In rowLayout.RenderColumns
            If TypeOf c Is GridViewRowHeaderColumn OrElse TypeOf c Is GridViewIndentColumn Then
                systemWidth += rowLayout.GetColumnWidth(c)
            End If
        Next

        Dim rowWidth As Integer = CInt(rowLayout.DesiredSize.Width) - systemWidth

        If settings.FitWidthMode = PrintFitWidthMode.NoFitCentered Then
            currentX += ((drawArea.Width - rowWidth) / 2)
        End If

        Dim fi2 As FieldInfo = GetType(TableViewDefinitionPrintRenderer).GetField("firstPage", _
                                                                                  BindingFlags.Instance Or BindingFlags.NonPublic)
        Dim firstPage As Integer = fi2.GetValue(Me)
        If ((Me.GridView.ShowColumnHeaders AndAlso (firstPage AndAlso pageNumber = 1)) OrElse settings.PrintHeaderOnEachPage) _
            AndAlso Not settings.PrintHierarchy Then
            Me.PrintRow(Me.GridView.MasterView.TableHeaderRow, rowLayout, settings, currentX, currentY, graphics, _
                        drawArea)
            Dim rowHeight As Integer = Me.GetDataRowHeight(Me.GridView.MasterView.TableHeaderRow, rowLayout) + Me.GridView.TableElement.RowSpacing
            currentY += rowHeight
            drawAreaHeight -= rowHeight
        End If

        fi2.SetValue(Me, False)
        Dim skipPageCurrentY As Integer = currentY
        Dim row As GridViewRowInfo = Nothing

        If Me.PrintPages.Count > 0 AndAlso Not settings.PrintHierarchy Then
            row = traverser.Current
        End If

        Dim firstRow As Boolean = True

        While traverser.MoveNext()
            If Not (TypeOf traverser.Current Is GridViewDataRowInfo OrElse TypeOf traverser.Current Is GridViewGroupRowInfo OrElse _
                    TypeOf traverser.Current Is GridViewSummaryRowInfo OrElse (TypeOf traverser.Current Is GridViewTableHeaderRowInfo _
                                                                               AndAlso settings.PrintHierarchy)) Then
                Continue While
            End If

            Dim hierarchyRow As GridViewHierarchyRowInfo = TryCast(traverser.Current, GridViewHierarchyRowInfo)

            If hierarchyRow IsNot Nothing AndAlso hierarchyRow.Views.Count > 0 Then
                Select Case settings.ChildViewPrintMode
                    Case ChildViewPrintMode.PrintFirstView
                        hierarchyRow.ActiveView = hierarchyRow.Views(0)
                        Exit Select
                    Case ChildViewPrintMode.PrintCurrentlyActiveView
                        'Do nothing the view is already selected
                        Exit Select
                    Case ChildViewPrintMode.SelectViewToPrint
                        Dim e As New ChildViewPrintingEventArgs(hierarchyRow.Views.IndexOf(hierarchyRow.ActiveView), hierarchyRow)
                        Me.OnChildViewPrinting(e)
                        hierarchyRow.ActiveView = hierarchyRow.Views(e.ActiveViewIndex)
                        Exit Select
                End Select
            End If

            rowLayout = Me.GetRowLayout(traverser.Current, settings.FitWidthMode, settings.HierarchyIndent, drawArea)

            Dim rowHeight As Integer = 0

            If TypeOf traverser.Current Is GridViewGroupRowInfo Then
                rowHeight = Me.GetRowSize(traverser.Current, rowLayout).Height
            Else
                rowHeight = Me.GetDataRowHeight(traverser.Current, rowLayout)
            End If

            If (currentY + rowHeight >= drawArea.Bottom OrElse skipPageCurrentY + rowHeight >= drawArea.Bottom) AndAlso Not firstRow Then
                traverser.MovePrevious()
                skipPageCurrentY = currentY
                skipPage = currentPage <> pageNumber
                fi.SetValue(Me, currentPage + 1)

                If skipPage Then
                    Continue While
                Else
                    Exit While
                End If
            End If

            If TypeOf traverser.Current Is GridViewGroupRowInfo Then
                If settings.PrintGrouping Then
                    If currentPage = pageNumber Then
                        Me.PrintRowWideCell(traverser.Current, rowLayout, settings, currentX, currentY, graphics)
                        currentY += rowHeight + Me.GridView.TableElement.RowSpacing
                    Else
                        skipPageCurrentY += rowHeight + Me.GridView.TableElement.RowSpacing
                    End If
                End If
            Else
                If TypeOf traverser.Current Is GridViewSummaryRowInfo AndAlso Not settings.PrintSummaries Then
                    Continue While
                End If

                If currentPage = pageNumber Then
                    Me.PrintRow(traverser.Current, rowLayout, settings, currentX, currentY, graphics, _
                                drawArea)
                    currentY += rowHeight + Me.GridView.TableElement.RowSpacing
                Else
                    skipPageCurrentY += rowHeight + Me.GridView.TableElement.RowSpacing
                End If
            End If

            If drawAreaHeight < rowHeight AndAlso firstRow Then
                fi.SetValue(Me, currentPage + 1)

                Exit While
            End If

            firstRow = False
        End While

        If Me.PrintPages.Count > 0 AndAlso Not settings.PrintHierarchy Then
            If currentY + Me.GetDataRowHeight(traverser.Current, rowLayout) < drawArea.Bottom OrElse _
                skipPageCurrentY + Me.GetDataRowHeight(traverser.Current, rowLayout) < drawArea.Bottom Then
                fi.SetValue(Me, currentPage + 1)
            End If

            Me.CurrentPrintPage += 1
            Me.CurrentPrintPage = Me.CurrentPrintPage Mod Me.PrintPages.Count

            If Me.CurrentPrintPage > 0 Then
                If row Is Nothing Then
                    traverser.Reset()
                Else
                    traverser.GoToRow(row)
                End If
            End If
        End If
    End Sub
End Class

WORKAROUND:
1. Create a custom GridTableElement and override the ProcessColumnEvent method.
2. Create a custom TableViewDefinition and override the CreateViewUIElement to return the custom table element.
3. Assign the custom view definition to the grid's ViewDefinition property

public class CustomGridTableElement : GridTableElement
{
    protected override GridViewEventResult ProcessColumnEvent(GridViewColumn column, GridViewEvent eventData)
    {
        if (eventData.Info.Id == KnownEvents.PropertyChanged)
        {
            RadPropertyChangedEventArgs args = eventData.Arguments[0] as RadPropertyChangedEventArgs;

            if (args.Property == GridViewColumn.IsVisibleProperty)
            {
                ViewElement.UpdateRowsWhenColumnsChanged();

                if (this.GridViewElement.AutoSizeRows)
                {
                    foreach (GridViewRowInfo row in this.ViewTemplate.Rows)
                    {
                        row.Height = -1;
                    }

                    this.UpdateLayout();
                    this.RowScroller.UpdateScrollRange();
                }

                return null;
            }
        }

        return base.ProcessColumnEvent(column, eventData);
    }

    protected override Type ThemeEffectiveType
    {
        get { return typeof(GridTableElement); }
    }
}


public class CustomTableViewDefinition : TableViewDefinition
{
    public override IRowView CreateViewUIElement(GridViewInfo viewInfo)
    {
        return new CustomGridTableElement();
    }
}


this.radGridView1.ViewDefinition = new CustomTableViewDefinition();

To reproduce:

this.radGridView1.EnableFiltering = true;
this.radGridView1.Columns.Add("Test column");
this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
this.radGridView1.Rows.Add("Test ");

Workaround: use custom filtering: 

this.radGridView1.EnableCustomFiltering = true;
this.radGridView1.CustomFiltering += radGridView1_CustomFiltering;


private void radGridView1_CustomFiltering(object sender, GridViewCustomFilteringEventArgs e)
{
    if (this.radGridView1.FilterDescriptors.Count > 0)
    {
        GridViewCellInfo cell = e.Row.Cells[this.radGridView1.FilterDescriptors[0].PropertyName];
        e.Visible = cell.Value.ToString().StartsWith(this.radGridView1.FilterDescriptors[0].Value.ToString());
    }
}

How to reproduce: 

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
        this.radGridView1.Columns[0].DisableHTMLRendering = false;

        this.radGridView1.PrintCellFormatting += radGridView1_PrintCellFormatting;
    }

    private void radGridView1_PrintCellFormatting(object sender, Telerik.WinControls.UI.PrintCellFormattingEventArgs e)
    {
        e.PrintCell.EnableHtmlTextRendering = true;
    }

    private DataTable GetData()
        {
            DataTable dt = new DataTable();

            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("Date", typeof(DateTime));
            dt.Columns.Add("Bool", typeof(bool));
            dt.Columns.Add("Description", typeof(string));
            for (int i = 0; i < 10; i++)
            {
                for (int j = 0; j < 5; j++)
                {
                    dt.Rows.Add(@"<html><b>Name " + j, DateTime.Now.AddDays(i), i % 2 == 0 ? true : false, "Description " + i);
                }
            }

            return dt;
        }

    private void radButton1_Click(object sender, EventArgs e)
    {
        GridPrintStyle style = new GridPrintStyle();
        style.FitWidthMode = PrintFitWidthMode.FitPageWidth;
        style.PrintGrouping = true;
        style.PrintSummaries = false;

        style.PrintHeaderOnEachPage = true;
        style.PrintHiddenColumns = false;
        style.CellPadding = new Padding(100, 0, 0, 0);

        this.radGridView1.PrintStyle = style;
        this.radGridView1.PrintPreview();
    }
}


Workaround: refer to the attached project

To reproduce: follow the steps defined in the following help article: http://docs.telerik.com/devtools/winforms/gridview/hierarchical-grid/tutorial-binding-to-hierarchical-data
Then, add a nested template in the second level. After closing the GridViewTemplate Collection Editor, you will notice that the template is not saved.

http://screencast.com/t/7VDYiopuUHn

Workaround: setup the hierarchy programmatically :  http://docs.telerik.com/devtools/winforms/gridview/hierarchical-grid/binding-to-hierarchical-data-programmatically

To reproduce:

private void Form1_Load(object sender, EventArgs e)
{
    this.productsTableAdapter.Fill(this.nwindDataSet.Products);
    this.radGridView1.BestFitColumns(BestFitColumnMode.AllCells);
}

private void radGridView1_ViewCellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
{
    if (e.CellElement is GridGroupContentCellElement)
    {
        e.CellElement.TextWrap = true;
    }
    else
    {
        e.CellElement.ResetValue(LightVisualElement.TextWrapProperty, ValueResetFlags.Local);
    }
}

Workaround: set the AutoSizeColumnsMode property to GridViewAutoSizeColumnsMode.Fill and use the ViewCellFormatting event to wrap the text: 

private void radGridView1_ViewCellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
{
    if (e.CellElement is GridGroupContentCellElement)
    {
        e.CellElement.TextWrap = true;
    }
    else
    {
        e.CellElement.ResetValue(LightVisualElement.TextWrapProperty, ValueResetFlags.Local);
    }
}

Until the new functionality becomes available you can use the workaround solution in the attached project.

To reproduce: run the attached sample project.

Workaround: set the AutoSizeRows property to false before exporting and set it back to true after the export is completed. 

To reproduce: follow the steps illustrated in the attached gif file.

One should be able to set the maximum row height when the rows are auto sized and the visual styles are exported.
 

To reproduce: setup the hierarchy at design time: http://docs.telerik.com/devtools/winforms/gridview/hierarchical-grid/tutorial-binding-to-hierarchical-data

Please refer to the attached gif file illustrating the exact steps.

NOTE: when the "Object does not match target type" error message is displayed, some of the properties are missing, e.g. MaxLength.

Workaround: modify the columns' properties at run time.

Please refer to the attached project.

Workakround: call the BeginEdit method in the RadGridView.Click event.

To reproduce: make sure only the first row is selected then Shift select row 3.
DataTable dt = new DataTable();

dt.Columns.Add("Value", typeof(int));
dt.Columns.Add("Date", typeof(DateTime));
dt.Columns.Add("Description", typeof(string));

for (int index = 0; index < 15; index++)
{
    dt.Rows.Add(new object[] { index % 5, DateTime.Now.AddSeconds(10), "Index = " + index });
}

radGridView1.DataSource = dt;
radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
radGridView1.MultiSelect = true;

foreach (GridViewColumn col in this.radGridView1.Columns)
{
    ConditionalFormattingObject obj = new ConditionalFormattingObject("Value",
        ConditionTypes.Equal,
        col.Index.ToString(),
        "",
        true);
    obj.RowBackColor = Color.SkyBlue;
    obj.RowForeColor = Color.Red;
    obj.TextAlignment = ContentAlignment.MiddleRight;
    obj.ApplyOnSelectedRows = false;
    this.radGridView1.Columns[0].ConditionalFormattingObjectList.Add(obj);
}

Workaround: use the CellFormatting event to apply the desired formatting by using the API for overriding theme settings: 

http://docs.telerik.com/devtools/winforms/gridview/cells/formatting-cells
http://docs.telerik.com/devtools/winforms/telerik-presentation-framework/override-theme-settings-at-run-time

To reproduce:

private void Form1_Load(object sender, EventArgs e) 
{ 
    this.productsTableAdapter.Fill(this.nwindDataSet.Products);
    this.radGridView1.EnableFiltering = true;
    this.radGridView1.ShowHeaderCellButtons = true;
    this.radGridView1.ShowFilteringRow = false;

    this.radGridView1.Columns["UnitPrice"].FormatString = "{0:$#,###0.00;-$#,###0.00;$0.00}";
}

Workaround:  in order to format the cells, use the CellFormatting event. As to the filter popup, use the following approach:
 this.radGridView1.FilterPopupInitialized += radGridView1_FilterPopupInitialized;

private void radGridView1_FilterPopupInitialized(object sender, Telerik.WinControls.UI.FilterPopupInitializedEventArgs e)
{
    RadListFilterPopup popup = e.FilterPopup as RadListFilterPopup;
    if (popup != null)
    {
        popup.MenuTreeElement.TreeView.NodeFormatting -= TreeView_NodeFormatting;
        popup.MenuTreeElement.TreeView.NodeFormatting += TreeView_NodeFormatting;
    }
}

private void TreeView_NodeFormatting(object sender, TreeNodeFormattingEventArgs e)
{
    decimal price;
    if (decimal.TryParse(e.Node.Text, out price))
    {
        e.NodeElement.ContentElement.Text = string.Format("{0:$#,###0.00;-$#,###0.00;$0.00}", price);
    }
}

To reproduce:
private void radButton1_Click(object sender, EventArgs e)
{
    radGridView1.Rows[0].Cells[0].Value = false;
}

private void Form1_Load(object sender, EventArgs e)
{
    radGridView1.AllowSearchRow = true;
    radGridView1.TableElement.SearchHighlightColor = Color.Orange;
    radGridView1.AutoExpandGroups = true;

    DataTable dt = new DataTable();

    dt.Columns.Add("test", typeof(bool));

    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);
    dt.Rows.Add(true);

    radGridView1.DataSource = dt;

    ((Telerik.WinControls.UI.GridViewCheckBoxColumn) radGridView1.Columns[0]).EnableHeaderCheckBox = true;
    ((Telerik.WinControls.UI.GridViewCheckBoxColumn) radGridView1.Columns[0]).Width = radGridView1.Width - 50;

    radGridView1.TableElement.Update(Telerik.WinControls.UI.GridUINotifyAction.Reset); //force header checkbox to check

    radGridView1.TableElement.ScrollTo(15, 0);
}

Workaround:
private void radButton1_Click(object sender, EventArgs e)
{
    radGridView1.Rows[0].Cells[0].Value = false;
    radGridView1.TableElement.Update(Telerik.WinControls.UI.GridUINotifyAction.Reset); //force header checkbox to check
}

To reproduce :
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();
        radGridView1.DataSource = GetTable();
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        var exporter = new GridViewPdfExport(radGridView1);
        exporter.FileExtension = "pdf";
        exporter.ShowHeaderAndFooter = true;
        exporter.LeftFooter = GridViewPdfExport.DatePrintedString;
        exporter.FitToPageWidth = true;
        exporter.PageMargins = new Padding(20, 15, 10, 10);
        exporter.RunExport(@"C:\Users\dkaramfi\Desktop\test123.pdf", new PdfExportRenderer());

    }

    static DataTable GetTable()
    {

        DataTable table = new DataTable();
        table.Columns.Add("Dosage", typeof(int));
        table.Columns.Add("Drug", typeof(string));
        table.Columns.Add("Name", typeof(string));
        table.Columns.Add("Name1", typeof(string));
        table.Columns.Add("Name2", typeof(string));
        table.Columns.Add("Name3", typeof(string));
        table.Columns.Add("Name4", typeof(string));


        table.Rows.Add(50, "Enebrel", "Sam", "Sam1", "Sam2", "Sam4", "Sam4");
        table.Rows.Add(25, "Indocin", "David");
        table.Rows.Add(50, "Enebrel", "Sam");
        table.Rows.Add(10, "Hydralazine", "Christoff");
        table.Rows.Add(21, "Combivent", "Janet");
        table.Rows.Add(100, "Dilantin", "Melanie");
        return table;
    }
}

Workaround:
Leave the default margins.

To reproduce:

Sub New()
    InitializeComponent()

    ThemeResolutionService.ApplicationThemeName = "Breeze"

    Dim dt As New DataTable
    dt.Columns.Add("Id", GetType(Integer))
    dt.Columns.Add("ParentId", GetType(Integer))
    dt.Columns.Add("Name", GetType(String))

    For index = 1 To 5
        dt.Rows.Add(index, 0, "Parent" & index)
    Next
    Dim rand As New Random
    For index = 6 To 25
        dt.Rows.Add(index, rand.Next(1, 6), "Parent" & index)
    Next
    Me.RadGridView1.Relations.AddSelfReference(Me.RadGridView1.MasterTemplate, "Id", "ParentId")
    Me.RadGridView1.DataSource = dt
    Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

    Me.RadGridView1.AutoSizeRows = True
End Sub

To reproduce:
 private void Form1_Load(object sender, EventArgs e)
 { 
     this.productsTableAdapter.Fill(this.nwindDataSet.Products); 
     this.categoriesTableAdapter.Fill(this.nwindDataSet.Categories);

     this.radGridView1.DataSource = this.productsBindingSource;
     GridViewMultiComboBoxColumn col = new GridViewMultiComboBoxColumn();
     col.DataSource = this.categoriesBindingSource;
     col.MinWidth = 200;
     col.DisplayMember = "CategoryName";
     col.ValueMember = "CategoryID"; 
     this.radGridView1.Columns.Add(col);
     this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
     this.radGridView1.CellValueChanged += radGridView1_CellValueChanged;
 }

 private void radGridView1_CellValueChanged(object sender, GridViewCellEventArgs e)
 {
     RadMessageBox.Show("CellValueChanged. Value >> " + e.Value.ToString());
 }