Up To You
Hi there! You have to remember, DVD's contain a lot of information. If you imported a movie from a DVD onto your laptop at full quality, it would be between 4-6 GB's of storage.
I usually keep my movies at 800MB, that way it looks fine on the computer, and it looks alright on ATV.
If you want good quality, I would compress the files to 1.5GB's or so.
Hope that helps!