Your numbers are a little off.
Most cartoons on homestarrunner.com stick with an 11:8 aspect ratio, slightly wider than the 4:3 aspect ratio of standard definition television. (The Brothers Chaps have explicitly stated that they just went with the Flash default of 550×400 pixels.)
If you're looking for a really convenient way to make the toons take up the full screen, and you use Firefox, then you should check out the
Greasemonkey script maintained by Phlip. Besides the fullscreen option, there's a seek bar, wiki links, subtitles, and more.