700mb for a dvd movie is simply because that's what the piracy groups would fit them into so they could be burned to a standard cd. 350 for an episode of a tv show because it is half that.
my suggestion, pick up a copy of
handbrake and convert a couple of movies on your own, it's free, only takes some time. there are a number of pre-sets for quality (i use high profile for everything) and see how big your movies are, you can use
vlc to watch them on your system. think about how many movies you want to carry at a time and how long you'll go between syncs with a desktop. then you can figure out what size is going to work for you.