What does scientific research say about the relationship between penis size and attractiveness? Pop-culture often equates a larger penis with a more attractive penis.


Is there any scientific research examining the relationship between penis size and attractiveness ratings?
What are the reasons for any such relationship?


I've heard that symmetry is relevance to beauty, but what is the relevance of size.
If you are interested in scientific research, you may want to read Lever, Frederick, and Peplau (2006). From their abstract


  Views about penis size were assessed in an Internet survey of 52,031
  heterosexual men and women... Whereas 85% of women were satisﬁed with their partner’s penis size, only 55% of men were satisﬁed with their penis size, 45% wanted to be
  larger, and 0.2% wanted to be smaller. Satisfaction did not vary
  across age groups from 18 to 65. Men reporting a larger-than-average
  penis rated their appearance most favorably, suggesting a possible
  confidence effect of perceived large penis size.


They also summarise research on actual penis size (p.130):


  Combining the means and standard deviations reported by these
  researchers, we calculated that the mean erect penis length was 5.3
  in. (13.5 cm), with 68% of men measuring between 4.6 and 6.0 in. (11.7
  cm and 15.2 cm), 13.5% between 3.8 and 4.5 in. (9.7 cm and 11.4 cm),
  and 13.5% between 6.1 and 6.8 in. (15.5 cm and 17.3 cm); only about
  2.5% of men possessed a penis over 6.9 in. (17.5 cm) long, and 2.5% were under 3.7 in. (9.4 cm) long.


They then go on to discuss:


  Why are so many men dissatisﬁed with their penis size when the vast
  majority of women are satisﬁed with their partner’s penis size? Past
  research has suggested that one reason men overestimate the importance
  of a trait to women is that media marketed to men give greater
  emphasis to the importance of this trait than do media marketed to
  women (Frederick, Fessler, &amp; Haselton, 2005). For example, media
  marketed to men (pornography, popular magazines) may emphasize the
  importance of supersized penises, whereas media marketed to women may
  not. If individuals use media as a source of information about what
  physical features are important, than gender-differentiated media may
  mislead men about women’s actual preferences. Future research should
  examine in greater detail the degree to which men overestimate the
  importance of penis size to women and the reasons for this
  misperception.


Drummon and Filiault (2007) presents qualitative data in relation to penis size in the gay community.

Francken et al (2002) surveyed 170 sexually active women.
They found that 20% of women found the length of the penis to be important and 1% very important. 55% and 22% found length to be unimportant or very unimportant. The pattern of results for penis girth were similar but girth was slightly more important. The correlation between importance of girth and width was very large (r=0.71) suggesting that there may be some underlying individual difference in women about the important of penis size in general. 

References


Lever, J., Frederick, D. A., &amp; Peplau, L. A. (2006). Does size matter? Men's and women's views on penis size across the lifespan. Psychology of Men &amp; Masculinity, 7(3), 129. PDF
Drummond, M. J., &amp; Filiault, S. M. (2007). The long and the short of it: Gay men’s perceptions of penis size. Gay and lesbian issues and psychology review, 3(2), 121-129.
Francken, A. B., Van de Wiel, H. B. M., Van Driel, M. F., &amp; Weijmar Schultz, W. C. M. (2002). What importance do women attribute to the size of the penis?. European urology, 42(5), 426-431. PDF