What are the requirements to be a "big board"?

Heh, for that matter, what does it take to be a medium or large board?

Posts, threads, members? A combination of all of those?

Most of the boards I am a member on, I'd simply call 'average' but even then, I am not sure what even that is. So I thought if I could get some input, I'd have a better means to judge just where my board stands