Most of the larger ones are Atlantics.
As far as I know, yes, Tarpon can get big. However, most of the wild Indo-Pacific Tarpon I've seen in fishing sites have been at around the 2 foot mark at the largest. Keep in mind, these were caught in brackish areas, so I wouldn't know how large they would grow in the salt where food is more varied.
I'd assume the growth rate for Tarpon would slow rather dramatically in a freshwater setup when compared to a salt environment, if only due to differences in diet that would likely be provided.