How many GB do I need to download a rental movie from ITunes?

The description in the iTunes Store gives the size of the movie. If you are acquiring it from elsewhere, the information should be available.

Depends upon the length of the movie and whether it's HD or not. A HD video needs approximately 1.2 GBs for one hour.

Approximately 1.5GB for standard and 3GB for HD. Most HD movies seem to be under 4GB, but I would check before downloading.

Excellent. I just went back to the ITunes store and had a look and the info WAS there! I am a very new user of movie downloads. Thanks!