The following explanation assumes you have some knowledge of using keyframes in Vegas.
I assume that your small window is created using track motion.
Place a keyframe where you want to start changing the size, then place the cursor where you want the image to be full size (assuming the sync cursor is turned on)then right-click the box and choose ‘restore’. this will set the image to full size and make a keyframe at that point in time.
The result will be that the image will change size over the duration of the two keyframes.
Gary Kleiner
Learn Vegas and DVD Architect
http://www.VegasTrainingAndTools.com