Well, better be glad, or hope you do only have one. They breed outta control. They are livebearers and can have up to 50 or more at a time. But, as for the size department. It depends on the size of your tank, but, most of the time they tend to get around 2 inches. That's in a large tank. I have a 30 gallon, hundreds of trumpets and the largest is 2 inches. I do not believe they get much bigger then that. They are cool snails to have, they keep your gravel nice and clean, but beware, they will breed way outta control.
just a tip.
Lakota