Use attached to reproduce.

- There is still space at the bottom of the grid. 

Workaround:

class MCCM : RadMultiColumnComboBox
{
    protected override RadMultiColumnComboBoxElement CreateMultiColumnComboBoxElement()
    {
        return new MCCBElement();
    }
}
class MCCBElement : RadMultiColumnComboBoxElement
{
    protected override Type ThemeEffectiveType
    {
        get { return typeof(RadMultiColumnComboBoxElement);}
    }
    protected override Size GetPopupSize(RadPopupControlBase popup, bool measure)
    {
        var result = base.GetPopupSize(popup, measure);

        GridTableElement tableElement = this.EditorControl.TableElement;
        int height = 0;
        GridTraverser traverser = new GridTraverser(this.EditorControl.MasterView);
        RowElementProvider rowElementProvider = (RowElementProvider)tableElement.RowScroller.ElementProvider;
        while (traverser.MoveNext())
        {
            height += (int)rowElementProvider.GetElementSize(traverser.Current).Height;
            height += this.EditorControl.TableElement.RowSpacing;
            
        }
        return new Size(result.Width, height +1);
    }
}