@Nico.campbell I wouldn’t worry about size. Even the smallest of the schools you are considering is 60 percent bigger than your current school. The student body will be more diverse than your current school, too, giving you much more to explore and learn about each individual.
I wouldn’t assume that bigger schools are less likely to have cliques, either. Often they just have more varied and specialized cliques (and sometimes, not always, people cling harder to their groups for comfort if the school feels too big.) That’s true about coursework and ECs, too. You will find more variety at a larger school (though you are still limited by the number of hours in the day!)
If there is something you are already passionate about and excel at, you might be able to explore it much more deeply at a larger school and find peers who have already achieved similar levels of excellence. But if you are looking to try brand new things, know that those classes, clubs, or teams may already be saturated with expert kids who have been specializing since grade school. Make sure you don’t let that stop you 
BTW: I really enjoyed reading your rationale for wanting a larger school. We are in the opposite boat. Our girl has more than 330 kids in her grade in middle school (the magnet HS she would attend if she stays home has 900 per grade) and we are looking for something much smaller for her. Thank you for giving us your insider perspective on some of the drawbacks of smaller schools.