Is it true that when a boy's or man's penis is fully erect it is the same length as his hand?
No, it is not true. People have always looked for some way to guess the size of what is hidden down under. Proposed standards have been the size of a person's foot, the length of his hand, or even the length of his nose. While you might find some people where there is a rough correlation, there are plenty of others who don't match at all.
Snopes.com has a fair article on this, there are some minor flaws in their statements, but it does give a good assessment of the reliability. See "The Size of Things to Come."