There are several ways to work around this:
1) disable autosizing (set AutoSize to false)
2) move the RadWindow after autoziging finishes (see attached example for a demo):
			function OnClientAutoSizeEnd(sender, args)
			{
				var wndBounds = sender.getWindowBounds();
				//this is the case when the viewport is not sufficient for the RadWindow
				//so the RadWindow is as tall as the viewport
				if (wndBounds.height == $telerik.getClientBounds().height)
				{
					sender.moveTo(wndBounds.x - 9);
				}
			}

3) avoid autosizing for the subsequent page loads in the content page.
This can be done by setting AutoSize to false and additionally the following function attached to the OnClientPageLoad event will provide autosizing for the first load without positioning issues:
 			function OnClientPageLoad(sender, args)
			{
				if (!sender.hasBeenShown)
				{
					sender.autoSize(false);
					sender.hasBeenShown = true;
				}
			}

4)  use partial postbacks in the content page so that it is not fully reloaded and the autosizing logic is not fired again automatically.