I read recently a rather poor statistic on the ability of women to orgasm. Only 25% of women consistently reach orgasm during normal sex (according to Elisabeth Lloyd, in a review of 33 studies, and published in her book "The case of the female orgasm"). Since many women find it difficult to reach climax during sex, one would assume that a larger penis, which provides more stimulation (both clitoral and vaginal), would greatly help women reach orgasm, as long as it is not too uncomfortably large. If this is true, then penis size must be an important factor in any sexual relationship.
To answer the question "Is bigger better?" once and for all, I would like to see a study performed comparing the ability of women to orgasm to the size of their partner's penis, which could be easily performed. I would bet that those women that can reliably orgasm have partners with statistically larger penises. (Note that guys packing a larger penis are likely to be more "cock-sure" in bed, which might bias the result). Just my 2 cents worth...