Without knowing much about your situation it's hard to make suggestions, but I'll try anyway. Given that you said you like knife fish, I'd say a standard 180g (6x2x2') is an ideal start-out tank. Big enough to keep the vast majority of popular aquarium species (not all together, obviously) and big enough to house most knife fish. Quite likely to be able to find one second hand for a reasonable price, not so big your powerbill will double, not going to require a separate room or hours of work, easily set up with commonly available parts (FX5 or similar canisters, 3' light units, etc etc). With a 180g you could have an interesting oddball community with say a black ghost knife, spiney eel, bichirs, some more mellow cichlids, etc etc.
That said, trying to tell someone which fish they should keep is like trying to tell them what sort of woman to find attractive....