My cock was never too big. However, I've known cocks that were too big, and I can relate what was said, but from the point of view of the one who receives it. To put it bluntly, I have a very tight ass, and exceeding 15 cm can already be challenging for me, especially if it is thick. Normally, I would say that I can take 16-17 cm. Beyond that, it's debatable.
And while I have fun sexting here, I don't consider myself an enduring bottom, even if I love to be bottom. Perhaps because of unsolicited/violent sex I have experienced in the past, I need to be extremely relaxed. Otherwise it can be painful for me as well as for the top.
I have already experienced cocks over 20 cm, but the guy has to take his time, a lot of time. But does he even want to? He must allow a few hours ahead of him! And because I feel like I'm the problem, I become more tense, and therefore less able to take it... In short, a vicious circle!
Not to mention my personal case, patience is the key word, my big guys.
I know that with the excitement and adrenaline, we would want to go even faster and deeper. But really, make the person understand that it's not a big deal, don't show frustration that you can logically feel. Make the person as relaxed as possible, and you may achieve your goal.