My experience is that scrollbars are usually 20px wide.
When you are saying RadPane, are you referring to a pane in a RadSplitter?
This link discusses resizing a PanelBar, which might help you approach your Javascript solution
http://www.telerik.com/help/aspnet-ajax/resize_panelbars_width_to_the_width_of_the_widest_radpanelitem.html
helpful?