I've never actually measured my girth, but as a guess, I'm probably in the 5.5-6" range as well. It would be hard for me to tell my peak size because when I do full-on cum I'm told I get very noticeably larger at that instant in time. I suspect all guys do.
I am definitely more of a grower than a show-er. I think this surprises some women that see me soft and naked in both good or bad ways, depending on what they're looking for. As mentioned, a guy really has no control over his girth, so it can sometimes suck if you're too big.
I have actually seen profiles that said they're looking for guys with average or smaller equipment. There are probably just as many looking for XL though. Having the ideal cock size is kind of like picking the ideal boat. There really is no such thing because it depends on where you're putting it. A canoe on the ocean can be just as bad as a cabin cruiser in a stream. The canoe will always fit in the ocean, but the cabin cruiser will likely not fit in the stream ;-)