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.)
Don’t lie. Here are some things you can say:
“Never thought about it. But I have a messuring tape here somewhere”
“It’s average”
“My last partner enjoyed it”
“When aroused; It’s exactly x cm girth and y cm from root to tip”
gesticulate, indicating size
“everything is relative”
“People stare in the shower room”
“it’s vanishingly small”
“I always had an advantage in that area”
“It’s satisfactory”
“It’s .00000000000107 or about 10⁻¹² astronomical units”
“It can make the Kessel Run in less than 12 parsecs”