An article currently published online in the Archives of Sexual Behavior found that there is a relationship between penis size and the likelihood that gay men will have unprotected anal sex. Specifically, they found that men who had engaged in unprotected anal sex were significantly more likely to have penis sizes that were larger than average, and that such men reported having significantly more trouble finding condoms that fit. Discomfort caused by condoms that were "too tight" made these men less willing to use them for safe sex.
These results suggest that research dollars spent on manufacturing more comfortable condoms - in a range of sizes - might be money well spent, in that widespread availability of condoms in a range of sizes might make this high-risk population more likely to use them. Previous research has also suggested that having a wider penis is associated with condom breakage, which means that a larger range of condom sizes may also be a safer range of condom sizes.
Because it amused me, I will also mention that another thing the study results suggested was that men spend a lot of time measuring their penis size. Before the study, almost 94 percent had measured their penis length, and 77 percent had measured their penis girth. Although the authors pointed out that the frequency of penis measuring may have been due to the population they sampled, i.e. highly-social gay men, I have to wonder if it would really have been that different in the general population. My impression is that it's just something that many guys do.