RadWindow does not support responsive size change and it does not change size automatically according to the viewport, because the behavior in such a scenario cannot be strictly defined - what percentage should it use, should it reposition, should it center, how should it handle the case when restriction zones or offset elements are involved? All these questions will have as many answers as many applications and business needs there are.
Thus, the developer would need to handle this resizing and apply the desired logic. Perhaps the following KB article will offer nice ideas that you can build upon: http://www.telerik.com/support/kb/aspnet-ajax/window/details/setting-size-in-percent-for-the-radwindow.
On the RadPageLayout demos - you can find an entire sample application built on top of it here: http://demos.telerik.com/responsive-web-design-aspnet/ and you can download its source in order to inspect it: http://demos.telerik.com/responsive-web-design-aspnet/source-code-download.zip.