Your favorite Scrum book will say "seven - give or take two". If you look at group theory (such as FIRO) you'll see things like "five through eight" or "if the group is more than ten people you'll have smaller groups forming within the group". Another interesting way to decide on team size I read about is the "two pizza rule"; If you can't feed your team on two pizzas or less the team is to big.

I think this a great way to describe maximum team size since it's funny and I guess it sticks. There might be a problem though since pizza size may vary. Two typical Swedish pizzas would not feed more than two or three hungry developers. If you go family size (or a typical US pizza I guess) you easily feed around five hungry developers. But I guess that is over analyzing the two pizza rule... Don't do that... Just buy two pizzas!