I'm sure this is a very interesting topic for many: Human Evolution and Penis Size. What is the history and did it matter for evolution? Read below to find out!
Human sexual selection
The sexual selection concept arises from the observation that many animals including humans develop features whose function is not to help individuals survive, but help them to maximize their reproductive success, which may include obstacles such as climate, availability of resources and competition from others for the female/male. This can be realized in two different ways: by making themselves attractive to the opposite sex (intersexual selection, between the sexes); or by intimidating, deterring or defeating same-sex rivals (intrasexual selection, within a given sex). We are all familiar with the concept of "alpha male" and ho they get all the chicks, however everything is a bit more complex than that. The German anthropologist Ferdinand Fellman argues that the emergence of human self-consciousness is due to an extended sexual selection, termed "emotional selection", bridging the gap between animal sexual behavior and human erotic love. Apparently even the human language may have derived to be more complex than other primate languages due to the romantic needs of the individuals through sexual display. It is not uncommon in the animal kingdom for some species to share sexual displays with humans, where the trait itself is not strictly sexual. Birds for example sing to attract a mate, and humans value voice in matters of sexual selection. Darwinists believe that the human brain itself, it being so 'unnecessarily' big and expensive to maintain in terms of survival shows that it is itself a sexual trait. Since humans are social beings, often the traits that are valued by the female are those that make men stand out in the social sense. Authority, respect, wealth and power are often thought to be correlated with what the female looks for in a mate.
Penis size is attractive but not necessarily important in long-term sexual selection
When it comes to physiology, the aforementioned Ferdinand Fellman had put forward a slightly modified model of "emotional selection" comparing to the traditional Darwinian model of "sexual selection" for humans, as the centre point of human evolution of consciousness (which brought about complex emotions). Part of the survival edge is due to a rare ability of humans to find mates for long term, which allows humans and females in particular to have feelings and feelings about feelings, therefore driving the proposed means of human intelligence and consciousness. Another proposed idea is that with a long-term mate, a being can save energy from competition in the future mating seasons and use that energy to provide for the offspring instead. There is a difference between attractiveness and looking for a long-term mate. Whilst studies in various countries have indicated that women prefer men with penises that are longer and thicker, there is no evidence to suggest that when looking for a mate a penis size matters. However, since human penis size in relation to their body size is the largest comparing to any primate it may suggest it had played a role in sexual selection in some way. Attractiveness of a male in itself is a series physical, psychological and social traits and characteristics, so penis size on it's own is of very little importance when it comes to selecting males.
Too big to succeed?
The women considered taller men with a more masculine body type (indicated by a larger shoulder-to-hip-size ratio) and longer penis to be more attractive, but not without limits — there were diminishing returns for extreme size, and men with substantially larger-than average features were not found much more attractive than those with only slightly above-average features.
Study leader Brian Mautz, a biologist now at the University of Ottawa in Canada, says that there seems to be a ceiling effect for each trait — a point of theoretical peak attractiveness, beyond which women’s ratings will begin to decline. The team’s model predicts that the most attractive penis would measure 12.8–14.2 centimetres in its flaccid state. Mautz notes that this ideal size is relatively closer to the population average (of 9 centimetres) than are the predicted ideals for the other traits, implying that women prefer more extreme shoulder-to-hip ratio and tallness but less extreme penis size.
Size did not matter for evolution
It was argued that female choice has been one important sexual selection force responsible for shaping the size of the penis. However, this force has been relatively weak, because women do not consider the size of their partner’s penis to be the most important determinant of their sexual satisfaction because in ancestral human societies, sexual satisfaction had been a secondary concern, and because women had limited space to exercise mate choice. The recent evolutionary transition to post-industrialism and the associated strengthening of female choice, along with the importance individuals place on sexual satisfaction, has resulted in evolutionarily novel pressure on men to provide sexual satisfaction to their partners. This creates concerns and anxiety about size, which may be augmented by cultural biases that tend to overestimate the average penis size.
What about other primates?
The average penis length of the Silverback Highland Gorilla is ~2.5 inches. They seem to be able to hold on to a few females, do the job in half the time and they are about twice as muscular or "manly" as the most muscular manly men. One can admire the balls of the researcher who took the measurements. Practically speaking penis sizes don't really come into it and when looking for a mate human female will prefer a confident, socially adapt male which can get an erection rather than a social donkey even when he's got with a big floppy dick. Moreover, on average women in the West report approximately 8 to 15 sexual partners in their lifetime, including one night stands, whilst men report more, which is considered an exaggeration. In that time it is unlikely that a Woman can actively select a mate based on penis size, since the majority of men come within range of average. In reality it's a combination of other factors that contribute to the process of sexual selection rather than penis size, which although may be weighed - presents only a minor characteristic that is considered. In terms of sexual selection it is understood that women place more importance on some physical characteristics such as height and hip to shoulder ratio and social characteristics such as status, wealth and kindness.
Sources:
http://www.penissizes.org/penis-size-sexual-selection
http://www.nature.com/news/bigger-not-always-better-for-penis-size-1.12770
http://www.tandfonline.com/doi/full/10.1080/23311908.2016.1147933