"Can I ask you if you've ever measured it?"
Of course you did, almost every day. And it was always the same: Not even 4 inches. You lie: "no I haven't, I just know it is very small".
"Ok perfect then we will measure it here" She says. You start blushing "H-here, are you sure?".
"Yes I'm sure and then it is a good training for you to stay naked in front of a woman, now take off your pants"