Currently if you don't set Width and Height to the RadWindow control it tries to auto-size to its content, but as we don't want to measure with infinite size (because this will break the virtualized controls) we decided to measure it with some size relative to the size of the Silverlight plugin - 0.75 of its width and height. This acts like MaxWidth/Height. To overcome this you may set MaxWidth and MaxHeight to the RadWindow control to some big value like 10000 and this will measure it with this size.
About the other problem - if you are using the same window instance it is normal for it to keep the size the user picked up by resizing - resizing changes the Width and Height of the RadWindow control. What I would suggest you for this case is to clear the Width and Height before showing new content in the Window.
If you have further questions, don't hesitate to ask!
the Telerik team