You'd be fine with anything from 3-5 gallons for a final pot size. depending on the strain. I'd say go ahead and use a 5 gallon if space isnt a problem.
5 gallon- ak47 etc 3-gallon afghan kush
when i think of the ideal pot size for an autoflower, especially where space is an issue (as it often is when using a/f strains) i think of a 3 gallon, or potentially a bunch of em in six inch squares