Who would have thought that a flash mob can be 'too big.' But if it is breaking your venue, it is as bad as the ones that are stealing and hurting people, due to the damages and the people who can get hurt on accident. When I start to plan my flash mob, I will need to know what I am going to do and where I am going to do it and set an ideal size based off of that.
I had an idea for a video game themed flash mob, with people dressed as Mario or Luigi, or Pokemon, and play the theme song and dance to that. I don't know. Just a concept.