The longest I ever saw and play with -respecting obviously my limits ;-) - was around 16 inches* with regular thickness. This one was belonging to someone having Jamaican ascendance. Of course, the extra is of almost no service, and I was even afraid to move as we don't want to bend it by inadvertance and giving a fracture.
(and even if this is not the subject, the smallest was 1 inch in full erection.)
*I'm saying "around" as I did not take the time to really measure it. It could have been as long as 18" maybe.