that depends on the species. i believe Florida and Shortnose Gar rarely exceed 2 feet (correct me if im wrong)

a good catfish to start out with would be the Four-Lined Pimelodus, which stays at about a foot. also try the Pseudomystus species, which are tough to ID in pet stores but most species also dont exceed a foot.
however from what ive heard, Pseudomystus species, when large, have a habit of taking smaller and similarly sized tankmates at night and slamming against the sides of the tank, as strange as it sounds.