typically speaking you can fit 3-6movies per DVD+R (or -R) disc and keep quality 'dvd like'
any movie/video around 1 hour 30 min = 700MB (audio converted down to 128kbps MP3)
any movie/video around 2hours+ = 1400MB (and you use the .ac3 audio track from the dvd)
that's pretty much 'the standards'
or... typically speaking, 1 hour of video to 700MB keeps quality quite close to original DVD.... although depending on how picky you are you can get 'up to' 2 hours of video per 700MB. but past that quality will drop off pretty quick.... i basically consider 2 hours to 700MB the MAXIMUM you can get from XviD/DivX while still being 'decent' video quality.
p.s. they play well on a XBox that's modified (which is pretty much 100percent FREE if you know what to do once u got the basic XBox Console) to run XBMC (XBox Media Center)