I think you can consider the RadPageLayout control that was added in Q1 2014 to adjust the appearance of your forms across different resolutions: http://www.telerik.com/help/aspnet-ajax/page-layout-overview.html. You can see it empower this demo application whose source you can download and inspect too: http://demos.telerik.com/responsive-web-design-aspnet/.
There you will see that you can use the RadFormDecorator's elastic design capabilities to have your forms change size: http://www.telerik.com/help/aspnet-ajax/formdecorator-responsive-capabilities.html.
There are other controls that offer elastic or responsive layout options and you can find this information in their respective sections of the documentation (e.g., RadDock, RadTooltip, RadScheduler, etc.).
On the RadWindow size - it cannot have size set in percent because it cannot be clearly defined what its behavior should be when the viewport size changes. Thus, you can use its elastic capabilities (http://www.telerik.com/help/aspnet-ajax/window-responsive-capabilities.html) to increase its title element size, and combine it with the idea from this KB article on resizing it according to the viewport with some custom logic: http://www.telerik.com/support/kb/aspnet-ajax/window/details/setting-size-in-percent-for-the-radwindow.
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