Women are emotionally sensitive: Her guessing the length of your dick is blatantly something she would overestimate; if you asked her what she thought, she probably “guesses” (and rightly so) that it’s bigger than it is because this would boost your self-confidence and improve the relationship.
If your girlfriend asks you if she’s fat, even if she is, you will down play it for sure…just as she will overplay what she REALLY thinks about your dick length. Though, not all women are so “sensitive” :D
Another retort to the question would be: how the hell would she know the difference between 6” or 8”, or even 8” and 12”? Does she measure her dick every day like you? C’mon, bra.
These are two…maybe contrary….thoughts….that should preclude whatever response she gives when you pop (pun intended) the “Big question.” Or drop it on her (pun intended)