The amount of storage you'll need to download and store a 2 hour movie will depend on the quality of the 2 hour movie you'll be downloading to your computer.
But on average a 2 hour movie uses between 800 Megabytes to 1 GB of storage per hour of the movie so to download the 2 hour movie I would advise you to have at least 2 GB of storage on your hard drive to successfully download and store the movie.
But of course the more the storage you have available on your hard drive the better.