Women take note: it is testicular size not penis size that matters!

In man, the penis protrudes from the body in a prominent way.  Anthropologists have demonstrated that penis size, like plumage on a peacock, may confer a reproductive advantage to the well endowed man, as women see this as a sign of fertility.   However, testicular size is far more important.

The majority of a testicles volume is made up of seminiferous tubules, where sperm are made.  Men with very small testis have less of this machinery.   The normal testicle should have a long axis (length) of ~4.5 cm.   One can use a $5 caliper to measure a testicle.  

So if you want kids, find a mate with large testicles.