Steve Mallett opines on the conjecture that humans have a "social capacity" of about 150 people in a community before they face overload. He seems to find compelling argument from Gladwell's "The Tipping Point" and Johnson's "Emergence...". I'm not sure if I buy it, but it's worth pursuing the original sources.
Memo to self: Read "The Tipping Point" and "Emergence". Yes, I'm ashamed, but ignorance is correctable.