I like to ask people who brag about size
"what was the serial number on your condom?"
they reply
"I dont know!"
then i say
"its all the way at the base... you have to roll the condom all the way down... guess you dont have much to brag about after all"
another good one that caught me off guard once when a friend of mine introduced me to a lady
he said
"Meet my friend John..."
as i start to shake her hand he continues
"... most guys will walk up and stick their d*ck in, but John sticks his d*ck in and
then walks up"
thankfully this was one of those "one of the guys" type of ladies.