Per design, the button width of the Ribbon should be automatic. Never the less it is still possible to set the width to a button via the width attribute.
However, during resizing that width may cause issues, as there are certain assumption in the resizing code to speedup the process.
I suggest that you do not force width of the RibbonBar items.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed