The DVD-5 is around 4 and a half gigabytes. The DVD-9 is around eight and a half gigabytes. Then there's the DVD-5 "Purist Edition," which is also around 4.5 gigabytes, and there's a 1.5 gigabyte AVI and I think a 700 megabyte AVI.
The DVD-5 and DVD-9 are meant to be burned to disc and played on a standalone DVD player, while the AVIs are just video files meant to be watched on your computer. They're also rough-drafts, basically, IIRC.