I think a board is big, when the admin feels it is big.

Big to me is very different than what it might be to you. For example, my private big board admin forum is for forum owners with 100k or more posts. I feel that once a forum owner sees his forum surpass 100k posts, he has to be serious about his endevour. There are hosting issues to look in to, possible security issues, opportunity for monetization, and more at this point.
I own 3 soon to be 4 "big boards" according to Quentin, but I don't feel they are all big.