Scientific studies of the size of erect penises show that the average erection is a little more than 5 inches long.
Like any aspect of anatomy, though, people vary. On the small side of things a guy is considered normal if his erect penis is at least 3 inches long. On the long end of things, its rare for erect penises to be longer than 8 inches long.
As for girth, most men average 3.5 to 3.9 inches when erect.
Generally, size matters little when it comes to sex, and you should be able to satisfy your partner unless you fall into an extreme end of either side of the spectrum.
Even if you have a “micropenis,” the medical term for a penis measuring less than three inches when erect, or an exceptionally large penis, pleasure is still possible. Some people prefer greater girth over length and some people aren’t comfortable with a penis that’s too long, so there’s no “right size” — it comes down to your partner’s personal preference. And a measurement of your penis’ size is not a measurement of your virility.
Learn more about penis size in the below video: