Those pics are adorable!
I've got some magentas, and the older set are huge! I'd say ping pong to golf ball sized right now, while my newer guys are about marble size.
I've heard that they get to be about the size of a kids fist? I don't know how correct that is, but out of all the snails I've had they are definitely my favorites!
Have you had any trouble with your's being really dumb? I've caught a couple that got themselves sucked into my filter intakes. One was killed by it, the other has a cracked shell and was in shock for like 3 days, but has recovered now. (Looks like the shocked one has had a run in like that before, the previous owner had glued pieces of his shell together)