You only need to use the RadWindow's client-side API:
http://www.telerik.com/help/aspnet-ajax/window-programming-radwindow-methods.html. Once you have the reference (it is returned by radopen(), and you use it - the test1Wnd variable) you can use the setSize() method on it.
As for window.onresize - this is a standard browser event and what you have attached will result in a JavaScript error. It needs a function name or a variable of type function that will be its event handler, so you need to remove it. A RadWindow instance is not the same as the window object each browser frame has.