Willies are measured with a ribbon. The length is measured from above, from the pubic bone to the tip of the glans. Foreskin does not count.
The circumference is measured by a measuring tape around the willy.
The length is related to the body height, but also to the length of the fingers. With a smaller willy, all sensory nerves are closer together. With a very large willy they are more spread. A willy that is too big can be clumsy. A larger willy is often found attractive for visibility. However, a small one is generally nicer to handle.
It is not the case that a willy no longer changes in size after puberty. The glans can grow, especially if it is exposed more often. Any erection also helps. Those who have a lot of erections on average can also grow a little bit in willy length and diameter. Conversely, when using steroids, what bodybuilders do, balls and willy can shrink again. As the foreskin is more often behind the glans, it becomes wider and straighter.