When using DOCTYPE //W3C//DTD XHTML 1.0 Strict//EN and setting height to 100px 
RadWindow AutoResize=True gives problems in IE10 by adding unnecessary scrollbars, and in Chrome, FF and Safari the window is to high (approx. 55px too much).

Refers to forum entry: http://www.telerik.com/community/forums/aspnet-ajax/window/autosize-behavior-question.aspx
(I used the same sample project to replicate the problem).

 Please see attached screenshot and project.