As long as you have a decent amount of room behind your seating area, the size of the room isn't important when considering a 7.1 set-up. For example: you could be in a very large room but have your couch against the back wall; a situation which wouldn't lend itself to a 7.1 set-up. Is it worth going 7.1? I think so. There are three advantages to going beyond a 5.1-speaker set-up: localization, stability and envelopment.

Having speakers at your sides and separate speakers behind you means you can get more exacting localization in the surround field than you can with 2 surround speakers. Some surround content sounds better at your sides (e.g., ambient effects, like rain) while other surround effects sound better when they end up behind you (e.g., front to back fly-overs). Not so easy to do with only 2 surrounds.

The best part about the improvement in localization is that it comes better stability. For example: no matter where you're sitting on your couch, sound intended to come from behind you will always appear to come from there; not some side-ish, rear-ish direction. No magic involved, just a pair of speakers physically located behind you (makes it hard for those sounds to come from any other direction). Again, it's difficult, if not impossible, to get this level of stability with just 2 surround speakers.

Finally, 4 speakers will 'surround' you better than 2 speakers can. If you listen to music in surround, you'll notice the difference in envelopment. Also, movement around the surround field will will be smoother with the additional speakers.

As for movies to listen to in 7.1, the most obvious choices would be ones with EX/ES soundtracks, where a surround-back channel has been specifically encoded into the mix. There are about 120 or so movies with EX encoding; enough to keep you busy for a while. Here's a link to a thorough and well updated list of EX/ES DVDs.

Best,
Sanjay