My own cock is 5.75 inches long. I have never measured my girth, having gotten really bummed out to learn that my length falls short of 6 inches. Despite my unhappiness with my size, I do rather like the shape and contours of my cock -- I think that it is aesthetically pleasing. It is what might be called a model cock, a.k.a. a small representation of the real thing.
On second thought, to be the perfect model cock, it ought to be a bit thicker. So, the same basic shape but with, let's say, an additional 20% in girth. That new version would then be my revised model cock. To scale it up to the real thing, I would opt for enlarging this revised model in all dimensions until it was 9.25 inches in length. Nine inches would really be just fine with me, but I would not want there to be any room for quibbling about whether I might be a hair shy of 9 inches. So 9.25 would provide a margin of safety -- clearly over 9 inches.
With the increase of 20% in girth and then scaling everything up so that the new length would be 9.25 inches, that means I would be packing just about 6 times as much meat as I am packing now!
Anything longer than 9.25 would be unwielding I think, and I would not want to be so big as to be regarded as a freak. I just think that it would be great to be a guy with really big cock.