Biggest you can fit, something that maxes out at 2' lol, biggest you can comfortable fit, I'd say go no larger than 18" fish, depending on the species on whether or not it's a long whispered skittish cat. I always wanted a wyckii lol. Why not do some schooling species of cat? Like ornate pims, sun cats, 4-line pims, you can possibly do a few of each if filtration is up to par. A lot of people get bored with one fish. If you're looking for a fish that isn't gonna be comfortable then I'd say do one that is less active like a wyckii. That's my favorite catfish. Though there have been individuals caught up to 28-30" they rarely grow past 24", arapaimag didn't grow one past 24" in his huge tanks. One stopped at 22". Depends on the individual.