The attached gif file illustrates the behavior on my end.

Hello Martin,

Thank you for writing.

You can find below a sample code snippet demonstrating how to limit the form's size considering the group panel's width:

 
public Form1()
{
    InitializeComponent(); 
    this.SizeChanged += Form1_SizeChanged;
}

private void Form1_SizeChanged(object sender, EventArgs e)
{
    if (minimumSize != Size.Empty)
    {
        this.SizeChanged -= Form1_SizeChanged;
        this.Size = minimumSize;
        this.SizeChanged += Form1_SizeChanged;
    }
}

Size minimumSize = Size.Empty;

private void HScrollBar_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "Visibility")
    {
        RadScrollBarElement scrollBar = sender as RadScrollBarElement;
        if (scrollBar != null && scrollBar.Visibility == Telerik.WinControls.ElementVisibility.Visible)
        {
            minimumSize = this.Size;
        }
        else
        {
            minimumSize = Size.Empty;
        }
    }
}

private void Form1_Load(object sender, EventArgs e)
{ 
    this.radGridView1.GridViewElement.GroupPanelElement.ScrollView.HScrollBar.PropertyChanged += HScrollBar_PropertyChanged;
}