A couple will figure it out after a while. I don't think there is a scientific absolute. A woman may prefer guys my size for comfort, then meet a guy hung like a horse but more compatible and she will probably teach/coach him how not to be an "uncomfortable" fuck.
I know must guys are fascinated by the world penis, but the vagina is an equally impressive creature as it can expand and contract to accommodate different size objects such as the penis, the dildo and then the ultimate destroyer a "baby".
Lots of gifted guys here but haven't scene one with more girth than a small cranium
^^^What he said^^^. Having a big cock, I had to learn that when having sex with a new woman, I had to learn her body and what she was capable of. In the first few moments, I would gauge just how deep I can go before she winced in pain or discomfort. Sure, the initial penetration of a tight woman is always going to elicit some response, but it should fade in a matter of minutes once the real pumping begins.
I've said this before; a woman's vagina is designed to take cock - it just is. So her body will adapt to whatever is inside her at the moment. Since I'm not a woman, I can't say with any degree of certainty that a large is or isn't uncomfortable, I can only go by what I'm told either during or after we fuck. But I imagine the sensation of a large cock continually pumping into her is a different sensation than say her pushing out a baby. Yes, the baby is MUCH larger, but the sensation is "different". For instance, the sensation of my doctor having to check my prostate is very different than taking a large dump. One of those I can go my whole without doing....it's just different.
In my honest opinion, a cock can never be too big. But you'll have to take my opinion with a grain of salt since I crave to have an extremely large cock.