How big do boys' penises get during an erection?
The size of a boy's erect penis depends on several things.
First, the size of your penis will depend on your age and maturity. Even newborn baby boys will have mild erections, but they are more a stiffening of the penis than a full-blown erection. Once you enter puberty your body will grow in various ways, including your penis. On average the male penis will not its full size until four years after puberty is first noticed. For the average boy that is about age 15 to 16.
Second, genetics plays a major role in determining the final size of your penis. The length of a flaccid (not erect) penis varies due to temperature changes. The size of an erect penis is more consistent, though it too will vary slightly due to the strength of the arousal. Oddly, the size of your penis when flaccid does not determine the size of your penis when erect. For adult males, the average size is just over five inches; the majority of men are between three and seven inches in length when erect.