Over the past several years, many not-for-profits have begun to shrink their boards to sizes ranging from 9 to 12 people. Both board and staff leaders have argued that a smaller board is more productive and easier to manage. This model seems to be coming from the for-profit board where smaller boards are the norm (comparably) and chief executives are also the board chair (though this trend is changing as more corporate boards are choosing an independent director to be the chair of the board).
I would contend that the size of the board should be based on the amount of community engagement the organization wishes to have. If a not-for-profit organization does not require significant community engagement then it does not need to structure itself with a large amount of community interaction. However, if philanthropy is to play a role in the organization, then community engagement at the volunteer leadership level is essential.