Well, there is no right answer for that one. An erect penis at around five inches is pretty much the average, and almost all lengths are fine. There is a condition called the micropenis that for adults is defined as a penis below 3.6 inches when erect.
Normally, for a man, the perfect penis size is the size that his penis is, because during sex a lot of other things matter a lot more than penis length. Whether it be the partner's pleasure, or attempts to conceive penis length typically does not matter. Also, there are no clinically proven, non-surgical ways to increase penis size, so any anxiety that men feel about their size may end up making them vulnerable to unscrupulous people trying to sell them unscientific "solutions."
In fact, to hopefully help put this anxiety away we decided to put up a quick self-assessment here. All you need to do is measure your erect penis and this will tell you your penis size rank percentile on the basis of a scientific study.Find out your penis length percentile