A jock strap joke

A man goes into a spoting goods store and is waited on by an attractive ww
female salesclerk.He says "I'm looking for a jockstrap".
She asks "How big are you?"
He replies "I don't know".
She says "Are you this big" while holding up one finger.
"No,I'm much bigger than that"
She says "Are you this big?" while holding up two fingers.
"No, I'm a little bigger than that".
She then holds up three fingers and asks "Are you this big?"
The man says "Yes,that's about right".
She then puts those three fingers in her mouth and says "That'll be a 
medium!"