that depends on the species. i believe Florida and Shortnose Gar rarely exceed 2 feet (correct me if im wrong)and gars get even bigger
yes they are worth every penny.TSN have a MINIMUM tank size of 500 gallons. baaad idea.
LSN might work, as they reach about 15", but Marbled Pim reach about 2 ft.
a few other good, unique, but expensive options are the Tigrinus and False Tigrinus (Juruense) Catfish. problem is a young Juru can be up to $60, and a 6" Tig can exceed $200. however, both of these species have stunning markings when they mature, so if they can go the distance, i imagine them being worth every penny.