To reproduce: use the following code snippet:
public Form1()
{
    InitializeComponent(); 

    for (int i = 0; i < 10; i++)
    {
        this.radDropDownList1.Items.Add("item" + i);
    }

    this.radDropDownList1.ListElement.AutoSizeItems = true;
    this.radDropDownList1.DefaultItemsCountInDropDown = this.radDropDownList1.Items.Count;

    radDropDownList1.VisualListItemFormatting += radDropDownList1_VisualListItemFormatting;
}

private void radDropDownList1_VisualListItemFormatting(object sender, VisualItemFormattingEventArgs args)
{ 
    args.VisualItem.Font = new System.Drawing.Font("Segoe UI", 24F,
        (((System.Drawing.FontStyle.Bold))), System.Drawing.GraphicsUnit.Point, ((byte)(0)));
    args.VisualItem.ForeColor = System.Drawing.Color.Navy;
    args.VisualItem.TextAlignment = System.Drawing.ContentAlignment.MiddleCenter;
    args.VisualItem.ImageAlignment = ContentAlignment.MiddleCenter;
    args.VisualItem.TextImageRelation = TextImageRelation.ImageBeforeText;
    args.VisualItem.TextWrap = true;
}

When the pop up is opened, the vertical scroll bar is displayed, although the DefaultItemsCountInDropDown property is set to the Items.Count.

Workaround:

bool flag = false;
SizeF size;

private void radDropDownList1_PopupOpening(object sender, CancelEventArgs e)
{
    if (!flag)
    {
        flag = true;
        e.Cancel = true;
        this.radDropDownList1.DropDownMinSize = new Size((int)size.Width, (int)size.Height * this.radDropDownList1.Items.Count + 1); 
        this.radDropDownList1.ShowDropDown();
    }
}

private void radDropDownList1_VisualListItemFormatting(object sender, VisualItemFormattingEventArgs args)
{ 
    args.VisualItem.Font = new System.Drawing.Font("Segoe UI", 24F,
        (((System.Drawing.FontStyle.Bold))), System.Drawing.GraphicsUnit.Point, ((byte)(0)));
    args.VisualItem.ForeColor = System.Drawing.Color.Navy;
    args.VisualItem.TextAlignment = System.Drawing.ContentAlignment.MiddleCenter;
    args.VisualItem.ImageAlignment = ContentAlignment.MiddleCenter;
    args.VisualItem.TextImageRelation = TextImageRelation.ImageBeforeText;
    args.VisualItem.TextWrap = true;
    if (args.VisualItem.DesiredSize.Height > size.Height)
    {
        size = args.VisualItem.DesiredSize;
    }
}