When I joined, there were about three major instances, and the rest were quite a bit smaller (I.e. <50 users). So I picked the smallest of the big instances (I think it had 500 or so users).
Basically, I wanted more than 100 or so users to have a better chance of the admin being active enough to respond to issues, but I didn’t want to be with one of the top instances to avoid outages and whatnot.