I really have no basis for a scientific answer. That stated, I can submit the following.
It must be long enough to inject Sperm to insure the continuing of the species. We do have a need to pro-create.
It should be side enough to initiate some pleasure for the female of the species so that she can enjoy sexual intercourse.
There are many ways to stimulate great sexual activity. From side to side, top to bottom and of course a plethora of positions for the couple to engage in to insure adequate penetration so that the sperm do not become lost in space (so to speak).
I am confident that a vast majority of males has the required equipment to meet the needs of pro-creation. We must also appreciate the learning curve to sexual enjoyment and pleasure. For me, life is a continuning educational opportunity. I look forward to learning something new each day, sometimes, even during ther nights.
Have a Great Day!!!!!