I have one Mystery snail, as well as a few other true mystery snails, that is, I don't know what they are. I am pretty sure I have a bunch of ramshorns, but I also have this one, perfectly round snail that I thought had been accidently thrown away, but I just found in a different tank. Might have slipped over there on a plant... Anyways, it is almost 2cm in diameter and perfectly round!
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?