Check out this brief post on the Signals v Noise blog. It cites the work of Edward Hall, who claims that the "perfect" group size is about 8-12. If you need more people than that, then your task is too complex and should be broken down further. Part of his reason is that above 12 people, maintaining effective communication with everyone becomes too difficult. People stop being "individuals" and get categorized. He also argues that leadership won’t develop naturally.