The "best size" for a committee or working group is the size that enables the group to get its work done effectively and efficiently. Thus, the best number of people for one project might be five, while the best size for another group might be 12. As a general rule, groups that have more than 18 to 22 people or less than five become more challenging to manage. However, there are many exceptions to that rule that work well.
Here are some tips that might help you to find the right size for your group: