Hello John Fairbanks,
RadComboBox does not support this functionality due to the fact that there is no way of determining, on the server, the font that will be used on the client (it can be defined inline, through CSS rules on the page, through linked CSS files and more importantly outside of the built-in skin) in which case the final width will be unknown.
You could implement a custom solution in your case, provided that you know the used font and calculate the width of the longest Item on the server based on that font.
On the other hand we will implement auto-width of the drop down on the client in future versions of RadComboBox.
the Telerik team