Sorry, that is because I have no experience at all with forums and their sizes, I just imagine that it will be significantly more than the average website, or am I wrong? - Maybe you can give just an idea instead, something like: How small would be a tiny little forum?
How big can a forum get? The only thing I can be sure is that the one I want to make would be anything in between the two - I know that is vague, but it will help already to give me an idea.