Currently, RadGridView does not support this functionality and there is no easy way to achieve it. However, you can check this article which demonstrates how to resize a regular Popup. You can try to use the same approach for our FitleringControl by predefining its template. Let me know if there is something I can assist you with.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.