Data rate times duration equals file size.

Use H.264 codec for most efficient results.

My favorite bit rate calculator for Mac. It's a widget.

http://www.apple.com/downloads/dashboard/calculate_convert/bitratepro.html
Roughly

675kbps times 6 minutes equals 29MB

The reasoning and the math is very simple. Even Compressor gives you an estimate file size for 1 hour of source material at a given data so all you need to do is figure out what that is per minute.