Definitions by Adolf The One and Only

5.1 inches

The average penis size on the world according to the first google result

Also see:
12.9 cm
1.5 credit cards
0.5 bycicle tires
2 Nobel prizes

Dude I heard Robert's penis is 5.1 inches
Really! Mine's only 2......Nobel prizes... O-of course.

5.1 inches by Adolf The One and Only May 25, 2017