I don't have a link handy, so you need to do a search for PAR or pixel aspect ratio
I think you use a PAR of .9 with NTSC 720x480 video to achieve 4:3 and a PAR of "something like" 1.2 with that same 720x480 video for widescreen
Also, for pictures, read...
Photo Scaling for Video http://forums.adobe.com/thread/450798
-Too Large May = Crash http://forums.adobe.com/thread/879967
-And another crash report http://forums.adobe.com/thread/973935