You're going to need a tape measure—not to measure the size of your dick (that appears to be irrelevant—to your fertility, anyway). No, no: it's the size of your taint that matters:

Men whose AGD is shorter than the median length—around 2 inches (52 mm)—have seven times the chance of being sub-fertile as those with a longer AGD, according to a study published on Friday in the journal Environmental Health Perspectives. That distance, measured from the anus to the underside of the scrotum, is linked to male fertility, including semen volume and sperm count, the study found. The shorter the AGD, the more likely a man was to have a low sperm count.

No word yet on whether promiscuity shortens your taint.