I went to the Winter Classic last year when it was in Miami, Ohio - just to watch, not to skate

It was a decent-sized competition. I think there were more skaters last year due to the Winter Classic title than there are normally at the Miami comp each year. I watched the FS 5 and 6 competitions, and I think there were about 5-6 skaters per group for those levels. They try to keep the groups small. What struck me the most was that there were tons of synchro teams last year.
ISI does divide everyone up into age groups by level, and sometimes also by gender, depending upon how many boys are competing at a certain level.