"I'll tell you, if you answer my questions first: How loose are you, deep are you, how long are your meat-flaps, do you shave, do you smell, and how wet do you get?"
or:
"Doesn't matter: I don't date women that go around asking guys how big their penises are."
or:
Hold up one hand with the index finger out and the thumb making a right angle... almost like you're indicating a size... but then hold the "L" up against her forehead and start laughing in her face because she's a loser.
One thing I learned from customer service: under promise, over deliver. I sometimes tell women I masturbate with an "elbow maccaroni noodle... uncooked."