QTPro if you have it. Otherwise, I think you can do this with the app I wrote,
metadata hootenanny (free). Drag the avi into it, click on the tracks icon in the lower right corner, find the video track, scroll right to the dimensions, edit them carefully, and save as a self-contained movie. Let me know if you have problems with it and I may or may not be able to solve them.
These apps will both transfer the video and audio bitstreams into mov files (not avi). I think this is your only bet, because avi doesn't have a mechanism for non-square pixels.