Most studies have a methodology section if you're curious about questions like this. The first study in the sidebar has much of the information behind a paywall. The second study has the full version available for free, but it's a study of studies, so you need to actually go through every study studied to get a clear idea of the exact methodology used.
From the first study, only some info available, much info behind paywall:
"A retrospective cohort study of 778 men (mean age 43.7; range 20-82) attending urological outpatient clinics in Saudi Arabia was conducted. Exclusion criteria were age under 18 years, a presenting complaint of small or short penis, Peyronie's disease or complaint of congenital curvature, clinical hypogonadism, and previous penile surgery or trauma."
From the second:
"Ovid Medline, Embase and PsychINFO were used to obtain separate literature searches up to March 2014. The results from the three databases were subsequently collated and duplicates removed. In addition, the authors inspected the reference sections of relevant papers retrieved through the database search.
The search strategy was:
Penis/ (Penis OR penile OR phallus OR genital).mp 1 OR 2 Organ size/ Size OR girth OR measurement OR length OR circumference OR dimension 4 OR 5 3 AND 6
Study Selection
The title and abstract of retrieved studies were screened by one author according to perceived relevance. The full-text articles of relevant studies were then reviewed by two authors and only included if they met the study inclusion and exclusion eligibility criteria.
Study key eligibility criteria: measurement of penis size by a health professional using a standard procedure; a minimum of 50 participants per sample. Exclusion criteria: samples with a congenital or acquired penile abnormality, previous surgery, complaint of small penis size or erectile dysfunction.
Limitations [note - excerpt] - It is acknowledged that some of the volunteers across different studies may have taken part in a study because they were more confident with their penis size than the general male population. Confidence to take part in size measurements may bias the measures to the larger end of the distribution. Equally there may be a bias towards the smaller end of the distribution if the full stage of genital development had not been reached in some men who had not reached the age of 18 years (or individual maturity), although this may depend on nutritional status and culture