I made a poll to try and ascertain an actual ideal size as selected by women themselves.
Women: Specify your ideal penis - Mister Poll
If you average out the results then the mean average ideal penis comes out at 6.6X5.3. It's also worth pointing out that:
A: an 8X6 penis apparently occurs in about 1% of men so; many of the 15% of women who chose this option probably have never even encountered one this size.
B: A penis that is slightly smaller than the ideal size will equate to a slight decrease in pleasure whereas a penis that is slightly too big will equate to a big increase in discomfort. When asked (forums, polls) what is "too big" women will often venture a size like 7 inches (6 inches was the ideal length for roughly half the respondents on my poll). When asked what is too small women will suggest something like less than 4 inches. In other words, the average size is already a lot closer to "too big" than it is to "too small".
C: Similarly a medium sized penis will be perfectly satisfactory to a more "capacious" woman but potentially painful for a "petite" woman. Again, slightly too big is more likely to lead to decreased sexual satisfaction than slightly too small.
All in all you have more to lose by aiming for "too big" than "too small", so we can probably set the ideal size slightly lower.
If you remove the 8 inch length and 6 inch girth votes then the mean average becomes 6.4x5.1 and this seems like a pretty reasonable ideal. In fact if I'd thought to include a 5" length option the average length would almost certainly have been a little lower.
Basically what I'm saying is that the average size and the ideal size are pretty much the same: Round about 6X5. It makes sense right? If the female orgasm is a natural part of sex then why would men evolve to be slightly too small to supply those orgasms? Surely they would evolve to be (on average) exactly the right size? It seems obvious that the average size and the ideal size really ought to be about the same.
If anyone is thinking of getting into PE I would advise 6X5 as a good goal size. I guess you can do what you want with your body, but would point out that out of the women who specified an ideal size only 14% chose 8X6. So if you're already average sized and aiming for a huge penis, you're probably less likely to satisfy the average woman with your goal size than you are with your current size. Food for thought.
Here's a summary of the results. I'll give the results in order of importance (i.e. the questions where the fewest respondents chose "no preference") and the share that the most popular option received from the respondents who did have a preference.
The Perfect Penis:
Foreskin: Circumcised (94% share of 99% of respondents)
Pubic Hair: Shaved or Trimmed (97% share of 99% of respondents)
Glans to shaft size ratio: Mushroom-shaped (96% share of 96% of respondents)
Length: Mean: 6.6, Median: 7, Mode: 6 (50% share of 73% of respondents)
Girth: Mean: 5.3, Median: 5, Mode: 5 (45% share of 72% of respondents)
Ball Hang: Higher than the tip of the penis (88% share of 62% of respondents)
Testicle size: Big (91% of 46% of respondents)
So the most important factors for women are foreskin and pubic hair (where their preference is overwhelmingly for circumcised and shaved/trimmed) and the least important factors are the size and position of the testicles.