Perfect Team Size

Bureaucratic Theory states that one person should manage no more than 8 people.

Web Design Teams

Many say the optimal size for a web design team is 3.

SCRUM Team

For SCRUM projects, the optimal team size is 7 +/- 2.

We've all heard of "magical 7" in the context of remembering phone numbers, etc. George A. Miller's The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information is a good reference point for understanding how that works.

For Getting Something Done

Edward T. Hall in Beyond Culture, beleives the optimal team size to complete a task is 8-12. If a task is too overwhelming for a team that size, it should be broken down into smaller components.