It all depends on the unknown catfish, if they stay small you could get a 55 gallon, if they get very large a 75 gallon might not be big enough. Some catfish get several feet long.
Depends on where you look, but...
Bala Shark - 10-12"
Common Pleco - 12-18"
Tetras - 2"x3 = 6"
Rainbow Shark -...