I would go with something outrageous, make a joke of it.

Q- "How big are you?"
A- "How big am I? I'm hung like a tuna can. I can only go an inch deep, but I wreck the hell out of the sides!"

I've actually used that one before and it's always good for some sort of a laugh.

Then, immediately cut thread. If she persists, ask her if she wants to find out...

_________________
Life is a game. Win.

"Funny Cocky stuff doesn't work here because its labeled as avoiding the question"

That's why you control the frame and change the subject... quickly I might add. You're right you never do answr this question, you simply laugh and move on. That's what you do with all shit tests. And actually yes, cocky/funny is what you have to use here. If you don't and you get all serious she will instantly lose interest in you. That's what a shit test is all about so actually that's all that will work in this case. It's the basis of the interaction. After you make her laugh you simply stack to the next routine. If she keeps getting back to the question use it in your favor to tease her about being so forward and sexual: "Is that all you think about?" "You just want me for my body" All of these must be delivered in a cocky/funny teasing nature. Set the frame that she is chasing you and she will in fact chase you. It's that simple.

O and if she really can't stop asking... just whip it out and show her. If she's a sure thing then hey why are you wasting time? If she's asking to see your cock she's giving you the green light dude.