Implement the ability to set the size of RadWindow in percents out of the box. The following knowledge base article describes how it could be achieved for the time being using the RadWindow client-side API: http://www.telerik.com/support/kb/aspnet-ajax/window/setting-size-in-percent-for-the-radwindow.aspx
As we have provided several workarounds to implement this functionality we believe that most people looking for this functionality are covered. However, as our configuration properties do not allow to be set to percentages without some additional code, and due to the limitation of the status options, we’ll mark this as ‘Declined’ for now.
You can also make it respond to small viewports by taking up their entire size by following this article https://docs.telerik.com/devtools/aspnet-ajax/controls/window/mobile-support/responsive,-adaptive-and-elastic-capabilities#fluid-or-adaptive-design-with-radwindow