[Pirate]

Penis size, like breast size, is usually more about visual impact than about functionality. That being said, women, I am told, can be quite sensitive to even quite modest changes in penis girth, reporting a greater feeling of fullness and, sometimes, more clitoral stimulation with a girthier partner. Changes in length is a different matter: a little deeper penetration is not noticed until the thrusting starts bumping up against the cervix, which most women find uncomfortable. With practice, and patience, some couples can learn to ease the glans of his penis past her cervix and enjoy full penetration, but that is more a matter of avoiding discomfort from a long penis than achieving more pleasure. One thing that a longer penis does offer is a great number of positions. Some positions are prone to the penis slipping out (standing positions or spooning for example) and a longer penis can give a couple more operational flexibility.
Of course, having too long, or too girthy, of a penis can also eliminate options as well if the penis in question is big enough to cause discomfort. This can be particularly true of oral or anal sex.
But the reason why most guys seem to want a big penis is so that other people will look at it and say "Wow, you have a really big penis!"

By the way, I love Diana Rigg (Emma Peel and Mrs. James Bond!)