The amount of data required to download a movie depends on the length of the movie and the quality of the movie you're downloading.
If the movie is an hour long movie and it's of standard quality then downloading that movie would be around 1 GB of data minimum.
If the movie you're downloading is an HD or other high quality movie and still an hour long it may use 1 and half GB of data or maybe even 2 GB of data to download the movie.