In a real camera, the size of the film back determines how wide a given focal length of lens looks. Like a real camera, Maya uses the relationships between focal length, sizes of objects, and the scale of the film back to determine how the field of view and depth of field should look.

You can probably use whatever film back setting you like unless you're working to match live-action footage, or unless you're trying to match animation from another software package, in which case it's critical to make sure that your settings match.

-- Mark