Is there a sweet spot when it comes to board size? Yes, there very likely is, but it differs for each board. One size does not fit all.
The board should determine its optimal size based on its needs. The primary guide for determining board size is the board’s function, which may change over time. Numerous factors influence the composition and thus the size of the board: board responsibilities, committee structure, legal mandates, phase in the organizational lifecycle, need for diversity, and maintaining a manageable group. It is impossible for an outsider to recommend a standard size for all boards.
However, it is difficult to imagine that a board with fewer than five members is able to incorporate all the desired qualities and capacity or that an exceptionally large board is able to engage every member in a constructive manner. Regardless of size, all board members must be engaged, as all are equally liable for the organization.
Wondering how to determine the best size for your board? This resource provides some tips.
Read more about understanding board sizes at BoardSource.