What features or statistics classify a board as a large board or a small board? Obviously starting a new forum, you're going to start off with a small board but how many members do you need to be considered a "big" forum? Does membercount matter, or is it all about post counts?
Depends on the niche. A popular niche needs more activity to be 'big' than a smaller, less popular one. For example, a one thousand member forum about the Wario series would be huge for that market, but a forum of the same size about general gaming would be medium or small.