Commercial DVD's range from 4-8+ GB. DivX/MPeg1 movies should be 100-700 MB+ in size.
You may need xtra codecs to play some of these movies; eg: Nimo's codec pack: http://www.divx-digest.com/software/nimo_pack.html Or alternatively, try another player such as VLC (it's free) or perhaps Hyplay (also free) or even this free one. But then, there's many around.