I thought they only get 3". I am positive it was a clown pleco, it even said it on the tank. The poor thing was all cramped up in the little 10 gallon box. Makes me worry about getting other "small" fish..
I certainly don't think it was a Clown Plec, as smithrc they only reach 5" at max, normally less. I'd say its most likely to be something like a Gibbiceps Plec?