There are various options for the level of compression. If it's a visually complex movie that you'll want minimal compression for (Star Wars, Lord of the Rings, whatever), you might choose just the movie without any options (subtitles, deleted scenes, etc). Something more dramatic where the visual experience isn't as important you can easily compress a 7.9GB video onto a normal DVD-R. Basically you just burn a few and compare your results to get a feel for how compression affects the picture, kind of like picking a level of compression in iTunes.
I believe that some of the other options out there like DVD2OneX are more versatile in terms of how much/little you can compress parts of the film to make it fit, but a quick trip to the website would clarify that.