Hey Rob
When you have your QuicktimePro movie open, go up to the top where the word WINDOW is, pull down to Show Movie Properites … then find the section in that window near the upper half called VIDEO TRACK… then highlight the words VISUAL SETTINGS. You will then see the option SCALED SIZE.
Put in 720 x 480 but make sure the Preserve Aspect Ratio is unchecked first…
Hope this helps!
Jonny