RadWindow has width and height properties which are applied with an inline style directly to the main wrapping element. If you do not have set width and height to the controls, it has 300x300px by default. If you want to apply it through style, you should use !important flag in order to overwrite the inline default styles, which are heavier. In your case you should modify the css in the following way:
All the best,
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!