Even in reputable studies, the statistical average varies, US median is about 5.5 inch according to the biggest study (1661 sexually active men) we could find by the well known Journal of Sexual Medicine. Most men have an erect size between 5 and 6 inch, so below 5 inch can be considered small. This data matches my own long term experiences as an andrologist. Don’t be fooled by the penis sizes you often see in adult movies, all actors are selected individuals far above the average size. These movies certainly don’t depict any statistical average and also use visual tricks to make the manhood of the male actors appear even larger. Nevertheless, many men ask themselves how to get a bigger penis the natural way, without any unnecessary risks and dangers.
Contrary to popular belief, your penis is not a muscle, nor does it contain a bone. It's better to think of it as a kind of sponge, which, when flaccid, is soft and malleable, but can quickly become engorged when filled with blood. Two large cylindrical chambers, the corpus cavernosum and the corpus spongiosum (see?), fill with blood during arousal, causing your penis to harden and grow in size.