The method evil_santa suggested is the best if you want precise dimensions, or you want to save the movie with its modified settings. There's a much easier crude method, however:
Open the movie in QT Player, hold down the shift key, and drag the resize corner. Instead of being held to the correct proportions, you can resize it to any proportions. The video will be stretched, not cropped.
Not precise, but a really quick fix for movies with the wrong proportions.