I don't think there is an easy answer.
Too small a forum will give a closer knit band of posters - but it will also make people leave faster and even make people avoid joining because the fora's dosn't look used enough.
Too big a forum will remove the knowledge of most users - but the most hardcore posters and contributors.
Personally I would rather have a large forum with plenty of users, cause I like to comment more on others posts than creating my own. I visit these fora's several times a day and many times there hasn't been any new postings - if it wasn't for my irrational passion for Chloë, I would have left and never came back long ago.
Chloë has several fan-sites and fan-fora's on the net - and none (maybe the russion, as Sergey has mentioned) seems that big - and often I have wished for these fora's to simply merge into this one (considering its the most official and the forum system is the best of them) to make a better Chloë fanbase discussion.
I actually think, that the "inactivity" on these fora's are stopping many people from joining - which ofcourse makes it harder to "recruit" new people, but thats the life of a online community. Its very hard to get it up and running - but once it kicks off it will substain itself and only need moderation.
That being said, I wouldnt really like a insanely huge fanbase where threads and posts disappear (to secondary pages etc) because of too many posts. I have no time to read through page after page of comments, because new posts are being made by the second.
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