Joel,

I've tried looking through various articles and PPT itself, but I don't see any options to start with one video size and have that same video resize itself for the subsequent slides. It's possible there may exist a custom macro to do this (maybe something you would input inside of Design mode) but I haven't found one. Additionally, I'm quite convinced that this is beyond PPT's abilities.

Possible Solution:
Edit your video, Trim the length for Slide 1, Save as a different file.
Use shortened video for Slide 1.
Use original video for Slides 2 - whatever. But in Animations>Custom Animations>Effect Options>Start Playing>From Time (seconds), start the video playback at the time where the first one left off.

This will probably create a visual break and not provide you with a seamless video playback transition from Slide 1 - 2, but hopefully it will help your situation for now. Perhaps placing a 'Fade to Black' and 'Fade from Black' at the end of video1 and start of Video2 will ease the transition on the eyes.

Video Editors: this may be a pain.
You can try your hand at Windows Movie Maker (usually comes with all copies of Windows).
If you have a friend with a Mac, perhaps they can edit it in iMovie for you.
Quicktime Pro for Windows is $30.

note:
Video editing is something that sounds straightforward enough when all you want to do is cut a video into two parts. But video codecs, containers, etc. can make your life quite stressful if one of these becomes problematic. Definitely try your free options first. You'll also want to be sure that your final video container output (video file type: avi, mpg, wma, etc.) is compatible with your PPT presentation. WMA files would be my best guess.

If you do find a native PPT solution to this problem, please come back and post. It'd be nice to know.