I have read that greater girth stimulates the G-spot and by that increases the lady's pleasure. A friend has a Bottle shaped penis, with a head about the usual size but a shaft that is much greater in diameter than the head (and anyone else's shaft that I have seen). He claims that when he inserts it in a lady's vagina, she usually has an orgasm in five seconds or so after the large diameter shaft is pushed inside her. He has many ladies contacting him out of the blue because his fame has spread by word of mouth.
As a bi-WM some years ago I made an appointment with a male escort who really did have almost 12", and girth a bit larger than usual, but not proportionally so. He was a super nice guy and very gentle, and eased it into me slowly and carefully and surprised me with how much of his great length managed to be inside me. He said that he could work it in so that the sigmoid colon was straightened out and lined up with rectum enough to receive his slowly inserted penis gradually. He thought that in some cases he might have gotten some of his penis into the descending colon, beyond the sigmoid colon. That of course suggests a radical straightening out and linear alignment of the rectum, sigmoid colon, and descending colon, even with some flexibility of the penis. It felt wonderful when he was fully inside of me (not the entire 12", but most of it). One or more of the valves in the colons seem to be sources of pleasure when something passes through them, even something as small as a colon tube. I have felt that pleasure most times with deep insertion of a colon tube, both during insertion and withdrawal. Incidentally, this nice man is well known and is engaged by clients all over the world, who pay to fly him to service them time after time.
The experience with that man suggests to me that great length may be of dubious desirability when inserted into a vagina, but with skillfull insertion it might yield great pleasure during anal intercourse. There is an entry on the internet that illustrates the expansion of the vagina and refers to it as "tenting."
Suzie has mentioned inserting her arm far enough into the colons to touch her lover's rib cage, perhaps she might be able to tell us something about the re-aligning of the various parts of the colon when an arm is inserted very far.