The average penis is between 4 and 6 inches.
If it were me, I would optimize my testosterone by working out with weights. I don't mean work out until you can't walk normally, but just add some nice muscle tone everywhere - arms, shoulders, back, abs, thighs, calves. That's how you naturally up your testosterone. Maybe that will cause extra size for you, and it certainly won't hurt you if you do it carefully with proper lifting technique.