I usually advise people to get a tent size for twice the number of the people that will be sleeping in it. Say, if two people are camping, then a four person tent would be best. Of course, this idea runs in to problems when the number of people grows pretty big, an in that case, I would advise increasing the number of tents.
The reason being is that usually its not just the people that are camping, but their stuff as well. So, that formula has always worked for us, having bigger tents than needed, because then everyone can sleep comfortable plus have room to keep their stuff too.
This thread also reminds me of a time me and three of my friends went camping. We are all pretty good size men, about 6 feet tall and between 200 and 250 pounds. So, definitely not the smallest of guys. One trip one of the guys had gotten a new tent and wanted to try it out, I didn't think to ask the size nor did anyone else, we just knew he said we should all fit in it just fine. That was one of the most miserable nights of camping I can remember. We had rode ATV's into a site, and were several miles from anything comfortable, it was raining so sleeping outside was out of the question. It was a very ill prepared trip, more of a spur of the moment thing. It was the four of us, in a smallish four man tent. We slept shoulder to shoulder, and anytime someone wanted to roll over it was a coordinated effort. You never heard so much complaining.
I have to say though, that trip is as memorable as it was miserable. I guess it's like that old saying "A bad day camping is better than a good day at work".