You can have a test at several genetic sites over the web for increasingly affordable rates which provide analyses and direct attribution of any related traits (they have already identified a great many genes, based on thousands of studies, which correctly predict hormone levels and other factors influencing penis size).
Disclaimer: This is not an advertisement, but I got a great many such results from a site called 23andme. I also had tests done for my parents and some of my siblings. It seemed to be fairly accurate in identifying my innate traits which I always have been aware of but haven\'t been able to pinpoint in origin (a recessive gene attributing me with greater than average height came from my mother; other genes which give me a tendency to exhibit a higher mathematical/reasoning IQ were passed onto me by both my mother and father; an SNP/marker called rs1799941 regulates sex hormone-binding globulin levels, and while forms AA and AG confer +27% and +15% higher SHBG levels on its bearers, me & all my family have the typical GG here which leaves my SHBG levels average and with the vast majority).
More interestingly & relevant to PE: marker rs722208 confers -3% and -9% (lower) levels of circulating testosterone in bearers of the AG and GG pairs, respectively (basically G/guanine in this SNP correlates with lower testesterone). These effects on testosterone are on men only, with the only effect on women being that they can pass it on. While my mother and male siblings have form AA (as do the vast majority of people -- leading to average and above average testosterone), my father carries AG, and I had a 50% chance to get a G-encoding from my dad, and I was the only son who did, leaving me with an AG here and thus an expected 3% lower testosterone level with all other things being equal. This, I believe, along with other (yet-to-be-identified) genes and environmental factors leading up to and during puberty affect one\'s penis size.