I think my max is probably around 8" or so?

If She Can Take It without Feeling Pain, Let Her Have It
 

I believe the answer lies in many factors such as the height of the woman, the positioning of both the cock and the pussy relevant to each other, and the importance of the woman experiencing several early orgasms to induce her cervix to open up to increase the odds of her becoming pregnant.

My hot wife is 5'-2" tall, and she managed to take two 9.5" cut cocks balls deep in her pussy. One alpha was a Black man, and the other was Native American (Cherokee Nation). Both men did her in the missionary position with a pillow under her hips. They both introduced their cocks into her very wet and very aroused pussy until they felt resistance. They would then begin a steady fucking motion until my wife experienced a very pleasurable orgasm. She said she could feel the entrance (cervix) to her mommy chamber begin to widen. The sequence was repeated until her lover's balls were resting on her asshole and she felt the cock head pass her cervix into her mommy chamber. Both lovers began to fuck in earnest until the alpha stiffened and with a scream of primal breeding conquest and accomplishment began to fill my wife with multiple, VERY hot streams of his baby-making goo. The deed had been done and nine months later my wife gave birth to her lover's baby.