The Width and Height properties of the RadWindow control are server properties that define its default size. If the control is disposed on the client and shown after a postback it will have these dimensions, regardless of the previous size it had in the browser (e.g. after the user had it resized) In your case the postbacks are most likely partial and the RadWindow is not disposed.
In the same manner you cannot know how large the browser viewport is on the server without adding some custom code that will store this information (e.g. in a hidden field) in order to pass it to the server.
So, what I an suggest is using the OnClientResizeEnd event
of the control and its getWindowBounds() client-side method
to get and store the current dimensions in order to use them on the server.
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