Balloon mollies get about 1.5-2 inches I believe. It looks like online it says 3 inches but mine haven't gotten that big before. The one I have with my angelfish (had it about 4-5 years) is about 2 inches I think and it looks HUGE to me. Maybe it's really bigger. 2.5 inches?
Anyways, they usually have about 20-30, but really big ones, can have maybe 50+ probably. It's just like guppies and platies. They are pretty similar. Mine get along fine with platies and guppies. I've had one that was somewhat aggressive before, but she calmed down. They aren't anymore aggressive than platies/guppies. (maybe a tiny bit more since they are larger, but not really)
They are just like guppies and platies. They eat any fry whether it be their own or guppy/platy fry.
It's kind of hard to tell if they are pregnant because they are naturally fat looking, but you'll know when their belly gets even bigger. Also if you look down from the top of the water, their lower belly will be getting larger as they get further along on their pregnancy. (just like guppies and platies)