Hi Mia, I post a lot of video on Vimeo. Here's their compression recommendations.
http://www.vimeo.com/help/compression
As far as single/multi pass encoding - it's similar to compressor's dvd settings 90 min best or 90 minute fast. The computer scans each frame image to detect movement, color change. If you select single, it does the best it can on first pass. If you select multi, it has a chance to detect and reproduce your movement more accurately - but this does take longer. There are hardware encoders that allow 10 or more passes to really produce an accurate image. But remember, this doesn't necessarily effect file size.
Compression codec, Frame Size, Kbps and Frame rate effect your file size the most. H264 is extremely common for web movies (and is available to all QT users) but there are some Flash video compressors that you can buy that produce some very clean, small .flv files.
Hope this helps! Best of Luck!
Kind Regards,
Chris
