This is the way autosizing should behave to allow better performance and faster reaction. Also, before the entire page is loaded in the RadWindow there is no way autosizing can be invoked properly as the page does not have its final dimensions. The initial size you refer to is either the Width and Height properties set in the markup, or the default 300 by 300 pixels the RadWindow has.
What you can do to work around this behavior: when you initially open the RadWindow you can use only the setUrl() method to make it load the page you want. Then in the OnClientPageLoad event that is fired when this content page loads you can show it, as autosizing has taken effect. Note, however, that this will request the content page each time the RadWindow is reopened, while its default behavior is to cache the page when it is closed. It will also delay its showing, especially in a real scenario where network delay is an actual issue.
I am attaching a simple example of this behavior and you can see how it works with me here: http://screencast.com/t/PPKEzKv5N1P
. I hope this fits your needs.
the Telerik team
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