RadComboBox supports different sizing modes, based on the DropDownSizingMode property of the control - set it to UpDown to allow sizing in vertical direction or set it to RightBottom to allow sizing in horizontal direction. Use UpDownAndRightBottom to allow sizing in both directions.

     radComboBox1.DropDownSizingMode = Telerik.WinControls.UI.SizingMode.UpDownAndRightBottom;