Funny Dirty Joke

A guy goes into a drug store to buy condoms. The girl behind the counter says, "What size?"
He says, "I don't know."
She holds up a finger and says, "That big?"
He says, "Bigger."
She holds up three fingers and says, "That big?"
He says, "Smaller?"
She holds up two fingers and he says, "That's it."
She puts the two fingers in her mouth and says, "Medium."