This reads like a joke question, I know, but I actually mean it.
Are you supposed to answer this sort of suggestive question with honesty? Or is this generally meant to be answered in coy and or affirmation? Because I don’t want to ruin the mood but I also don’t want to unwittingly spoil her expectations. (FYI, I’m average sized.)
I’d say lying is not a great idea if its someone you’d like a sexual relationship with as they’re going to find out anyway. That said the right answer is all about context. You could spin it playfully and use the find out type of lines or stay mysterious with something like “a man can’t share all their secrets”. If you’ve got the confidence you can give them an accurate number or idea of size visually. I’d bet the confidence/delivery of the response matters more than the actual number for most people.