We have an informal group in North Wales for a few months now and have had several meets. All of them have been around 8 people. I find I prefer a small number as everyone gets involved and is sociable. Larger groups seem to encourage splinter groups who go off on their own within the confines of the permission, which seems a little counter productive to me.
When a page for the group was put up on facebook, I was surprised to find quite a few locals who weren't on the forum but were interested in meeting up. I say give it a go and get it advertised across the different forums and facebook. Just be careful you don't find that too many people want to come !