When you set the MinWidth and MinHeight for example 300 on both properties of the RadRibbonWindow you shouldn't be able to resize the ribbon window under these values. We have prepared a sample project demonstrating this behavior.
Give this project a try and let us know if we are missing something. If yes, you can modify this project to reproduce the described behavior in your post so we further investigate it on our side.
Do you need help with upgrading your AJAX, WPF or WinForms project? Check the Telerik API Analyzer and share your thoughts.