Well they get to about 10" so, i'd personally be happy housing in it in a tank with a 5x2 footprint. Then again that's not following rules, thats just what i think would give enough swimming room, you could probably do smaller.
The bare minimum size tank i would put such a pleco in is a 4ft long by 18inch wide tank, however the bigger the better- if you could get a 5-6ft long tank, that would be great .