The two smallest ATF species reported are Hydrocynus tanzaniae, and H. sonorum, the Tanzaniae was reported on another site as getting to 3' but fishbase says 10", a museum site I checked said the specimins they had were between 8-10", they are not common and I can find no record of importation. The H. somonorum is also apparently not imported and gets to 22".
The forskahli is recorded at 31", the brevis at 35", the vittatus at 42", and the goliath at 53"