A somewhat noted astrologer in the 1980s wrote that one cannot trust what astrologers write about penis size because most men only have one example to go by. - I liked this fellow a lot, but the humorous remark not only ignored that some real research was going on but also ignored the large number of straight women and gay men in astrology circles.
In the category of actual researchers, Gary Duncan had been collecting information on this topic for several years when he could get it along with the Kinsey-like sexual behavior surveys he conducted. I asked him what he'd found - the problem was that he didn't think he had enough data to publish findings. Nonetheless, the trends were pretty clear, and pretty simple. Per Gary (who was mostly interested in sign data in the study overall), Scorpio Suns and Moons had the largest and Taurus Suns and Moons had the smallest. (What this meant was that Scorpio and Taurus had statistically anomalous highs and lows for penises outside two standard deviations from average size - not that this sign placement was the only indicator.) Asked about other factors, he said that Mars foreground or closely aspect luminaries (primarily Sun) would replace the Scorpio luminary. If a long indicator shows, it cancels a short indicator (which probably take the form of "least amount of longness" rather than short-prone per se).
So the answer was simple: Mars (by luminary sign, solar and perhaps lunar aspect, and angularity) was the strongest phallic indicator; opposite indications were the opposite.
I don't have the original data, but we do have a lot of public examples of significant porn stars for whom there is abundant video evidence, and other celebrities (actors, musicians) for whom there are either leaked sex tapes or published reports by their partners. (There are groupie sites where young women compare notes on the celebs they've bedded, and they don't hold back.) We have full birth charts on several of these and birth dates on others - and, with only a rare exception like Shemar Moore, these charts hold up to Gary's preliminary appraisal. Here are some for which you can find data easily enough:
- Liam Neeson has a Scorpio Moon.
- Jay-Z has a Scorpio Sun.
- Tommy Lee has a Scorpio Moon.
- Frank Sinatra had a Scorpio Sun.
- John Holmes has Mars setting (0°41').
- Wilt Chamberlain has Mars on IC.
- Milton Berle had Mars widely rising.
- Willem Dafoe has Mars foreground (but widely).
- Simon Rex might have been wearing a prosthesis but at least has a reputation for being super-sized - he has Mars 0°51' from Dsc, conjunct Moon.
- Jonah Falcon, allegedly having the largest penis in the world, had Sun conjunct Mars within 2° no matter what time he was born.
- Lyndon Johnson had Sun conjunct Mars within 2° (and may have had Mars rising).