So Schonfeld and Beebe developed the stretched flaccid measurement, which they validated by having 150 of their subjects mark the length of their 'morning wood' erections onto provided un-marked strips of cardboard every morning. That's a lot of penises to measure, and the bulk of those subjects would not physiologically capable of attaining an erection as needed for examination.
They examined the genitals of 1,480 male subjects, raging in age from infants to 25 years old (125 were 18 or older) to determine growth patterns of the penis and testicles. While there had been a handful of penis size reports before it, the first significant modern study to examine penis size was conducted by William Schonfeld and Gilbert Beebe in 1942 in New York City.