Welcome back!
If all we are given is tank size (~8'x2'x2' ?) and a potential tank mate, I'd recommend a catfish that does not exceed 1' (average swimmer), less then that for an active swimmer, 1.5' for a sedentary catfish.
There are too many choices. You can filter the Cat-eLog on Planet Catfish by size and browse through hits and see what catches your fancy and is available / affordable.