I find movies are between 500mb and 700mb, with the main factors being the length of the movie and the quality of the picture. A 30 min TV show is usually around 50mb (same factors apply). I find that my 8 gb Nano is more than enough for movie watching on my commute to work and class. I would also factor in the fact that watching a movie on the ipod, while nice, is very draining on the battery so be prepared to recharge a couple times a day if you'll expect to be using it frequently.
