This feature is already available. Please set the KeepInScreenBounds property to true. You can observe its effect with the following page:
Without the property you will not be able to see the titlebar, with it the window will be positioned to the top left corner.
You can also test its functionality by making the window even bigger some time after it is loaded:
oWnd = $find(
With the property the window will not move, just the scrollbars of the browser window will extend.
I hope this helps you achieve the desired functionality.
the Telerik team