I know this question has been asked and answered and discussed and debated many, many times on LPSG. But I think there is always room for fresh discussion with new people, so I am replying anyway.
I don't think there is, or could possibly be, a perfect size. Just as cocks come in all shapes and sizes, so do vaginas, and although assholes may be a little less variable, I'm sure gay men display just as much of a variety of preferences as women do.
Some women just cannot accomodate a very large penis; others long to have one and feel their best when being fucked by someone well-endowed. Some people like long ones, some people like thick ones, and some people like them long and thick.
So, when I state my idea of the perfect cock, it is only my idea -- I think everyone's preference is as unique as they are.
I like cocks that are thick, but not all that long. Longer than 7 inches I cannot accommodate -- my husband is 7 inches and bottoms out, just barely being able to fit all the way in. I like the length he has, because he can pound me deeply and reach my very core, but if he was any longer, we would not be able to achieve full penetration. So any length over 7 inches, for me, is a waste.
My husband is 6 inches in girth, which is plenty thick for me -- almost too thick. It can make me very sore very quickly from the friction, which is a drawback to having so much girth. I hesitate to say I would prefer a cock different from my husband's, but in truth if it was slightly less girthy it would probably be more practical. I do, however, love the feeling of being split open whenever he enters me.
So, if a 6 inch girth is almost too much for me, an even larger girth would probably be way too much. Very thick penises also have one other disadvantage, in that they are very hard to suck without getting your teeth on them. You have to be very precise and very skilled. And I'm sure there is a size at which no amount of skill could keep my teeth away -- my mouth is only so big.
So, having said I don't really want to fuck or suck a penis bigger than 7x6 inches, I will say I absolutely adore looking at bigger cocks, and fantasizing about them. In my fantasies, I can accommodate anyone, and getting sore is optional.
My husband once said I am a size maven rather than a size queen, because I like to look but a lot of big cocks are more than I could take in real life.
One final comment on my ideal cock -- from experience I know that upward curves are very stimulating to me. I don't have a lot of experience with downward or sideways curves, so I don't know how they would feel. But I do think an upward curved cock, to me, is somewhat preferable to a straight cock, at least when it comes to vaginal stimulation. Upward curved cocks have one drawback, though -- they are extremely hard if not impossible to deep throat, at least in an upright position.