Five times the size of an average penis would give you a thirty inch penis. I'd imagine it would get embarrassing when your erection exposes itself between your cleavage. And then you might need a ball strap to hold onto your giant testicles, for support, but that's a different story.
Oh, and if you with your giant-five-times-the-size-of-normal-testicles gets busted and your nuts swell up and can't fit into your ball harness...as if they were big before, now busted and swollen! Huge!
Having a thirty inch one would mean to have an extra reservoir of blood in order to get it erect without falling unconcious.
It'd simply draw too much blood to be true...
Damn, if I saw you like that, I'd just HAVE to get in a couple shots. You'd see me coming and shield your nuts, so I'd just kick you as hard as I could square in the middle of your huge cock. Then, once you're on the ground whimpering, stomp on your balls. That should leave you with a couple nice, big bruises.