Women who were shown various computer-generated images of naked men consistently rated those with larger members as being most attractive.
While various studies have shown that taller men are generally considered more attractive, researchers found that penis length was as important as height in determining a man's sex appeal.
Perhaps worryingly for some men, there was no point at which increasing length and width started to lose their appeal, with results suggesting the "ideal size" – if it exists – must be larger than any of the 343 figures used in the study.
Penis length had a particularly strong bearing on the attractiveness of taller men, possibly because their height might produce an unfavourable "size contrast effect", and appeared to be especially important to larger women, researchers said.
The importance of being well endowed did dwindle after a flaccid length of three inches (7.6cm), however, which is below average according to a separate study of 3,300 Italian men.
Writing in the Proceedings of the National Academy of Sciences journal, the Australian researchers said their findings suggest that the choices of women may have caused men to develop larger penises during human evolution.
Women's inclination towards men with a larger manhood could be down to enjoying greater pleasure during previous sexual liaisons, or may be an "aesthetic preference", they wrote.
The study found that being tall and particularly having broad shoulders relative to the hips also played a significant role in men's attractiveness.
Although the factors are tied to one another the shoulder-to-hip ratio explained about 80 per cent of variability in men's attractiveness in the study, while height accounted for 6 per cent and penis size for 5 per cent.
The researchers, said their results "directly contradict claims that penis size is unimportant to most females".
Dr Brian Mautz of the Australian National University, Canberra, said: "We found flaccid penis size had a significant influence on male attractiveness. Males with a larger penis were rated as being relatively more attractive.
"Our results show female mate choice could have played a role in the evolution of the relatively large human male penis."