I tried your scenario and I was able to reproduce the problem. The issue is that the SizeToContent feature of the RadWindow control for WPF works only for the first time the RadWindow control renders. This is because it sets its Width and Height properties as a work-around for a bug in the WPF Window control (which we use as a host). We will try to find a better way to work around the other problem. Here is a PITS item which you can use to vote for the issue.
the Telerik team
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