You can't. :(

You can click on the video to make it a tiny bit bigger.. but that's all. I have heard a couple of times that GT is moving to a better quality format (640x480 video I think), so may be this will improve the size on screen.

I have a resolution of 1680x1050 on my primary LCD monitor but I have my LCD TV connected as well.. Since I generally play/practise standing up, I usually drag Firefox over to the TV screen and watch the videos on that. I have the TV set to somethingx768 so the video is much bigger. :) This is also handy for recording too as you might imagine. :)