There's ups and downs for both. When you're part of a really large forum, there's constantly someone replying to threads, and it's always active, which doesn't happen to often in smaller forums. But, with larger forums, barely anyone knows eachother and there's really no sense of belonging like there is in smaller forums. I've been a part of both small and large forums, and it's really hard to choose which I like better.
Edit: I decided to choose medium sized forums, like LPF.