YouTube.com is a widly popular video sharing site. I recently wanted to upload some videos and thought I would share some info I have found.
The maximum size of the video can be 100M. YouTube accepts MPEG, MOV and AVI and I think MP4.

Once you upload the video YouTube reencodes the video into a Macromedia Flash Streaming format(LFV) which is displayed at 425X350. However the actual size of the video is 240X180. This makes the quality suffer a bit.

It seems like you can upload 320X240 or 720X480 and the final quality is not affected either way. I also tried some experiments with MPEG versus MOV and again I could not really see any difference in the final product. I think the sound quality was a little better when I did the initial encoding in MPEG.