First, how big should the board be? Well, I'm reminded of the old joke that asks, "How long should a person's legs be?" The answer is, "Long enough to reach the ground!" The related answer to, "How big should the board be?" is, "Big enough to get the job done." Then you might ask, "What job?" It seems to me the answer to that is that the board must make sure the organization is working to fulfill its mission. That means managing the executive director (or president in some cases), being sure the professional leadership is focused on the mission, and assuring that the organization has the resources (money, staff, supplies, etc.) it needs to fulfill that mission. When we talk about the board's "fiduciary duties" we're usually talking about resources.

So, one of the answers to the "how big a board" question must be, "Big enough to close an emergency budget gap." This leads to at least part of the answer to the question about managing diversity on a non-profit board.
There is no question that diversity of a non-profit board is crucial, especially if the organization's mission involves working with or supporting diverse communities. The board should look like its community. But diversity can (and should) also mean diverse skills and backgrounds. And in every case, fundraising ability (both giving and getting) must be a primary consideration for board membership. If the board doesn't have the ability to get the organization through a financial crisis, then the organization has a very high risk of not surviving. If the board believes in the mission, they should be both willing and able to support it! If they don't believe in the mission enough to support it financially, maybe they shouldn't be on the board.
This isn't to say that there should never be anyone on a non-profit board who can't make a major gift. However, if you aren't paying attention to fundraising as you populate your board, you run the risk of the organization failing. The most wonderfully diverse board in the world - however you define that diversity - will fail the organization and the mission if it can't give or raise the money to fund the mission.
So, how do you manage diversity on a non-profit board? Use a matrix to figure out what skills, education, ethnic groups, geography, and other diversities you need, and always remember the importance of fundraising to the success of the organization.
How big should the board be? How diverse should the board be? Big enough and diverse enough to succeed.
If you'd like to talk about ways to build and/or diversify your board, give me a call.