I don't know the exact size, but it would need to be large enough to provide a steady income for at least one teacher, plumber, butcher, nurse, vegetable farmer, electrician, seamstress, inn owners, dairy and chicken farm, bus driver, baker, fire dept. chief, music band, etc.

500-2000 people, perhaps.

Someone like a priest or philosopher, lawyer, engineer, scientist, professor, administrator, would still have value, but you wouldn't need one of each in every human-cluster. We probably have way too many of them, now, and too few plumbers and cooks.