I was unable to reproduce this behavior on my end. You can find my test page attached and a video from my experiment here: http://screencast.com/t/VgRPiqc0F99. As you can see the size does not change and neither do the values in the image editing software (please allow the few pixels error of my hand, though :) ).
Am I missing something? Can you reproduce this issue with my page? Is your setup different? If so please provide some more details on the matter so we can investigate it further.
Please note that for the AutoSize to work correctly you need to have explicitly set dimensions in the RadWindow's content and they need to be in pixels, not percent.
the Telerik team