That's tough. I've worked clubs where I've been the biggest and where I've been the smallest.
I think a lot of it has to do with "the look". You definately have to be in shape and look strong, intimidating, but nice too. Hard to describe. It's like, be nice, say "have a good time", and at the same time, let them feel that if anyone fucks up....well.....
Also, some clubs I've worked at have had a reputation of keeping things in order. That when someone fucks up, it's resolved very swiftly, and if they fight back, they end up bloody and in the rear seat of a cop car.