The RadWindow is actually an IFRAME element and you should use exactly the same logic as you would do with a standard IFRAME to get the same result. The control also provides the needed API for setting its sizes. You can find one sample solution here. We have also planned to introduce e property which when is set will make the RadWindow automatically resize according to its content but I cannot provide you an exact time estimate.
All the best,
Svetlina
the Telerik team
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