It's not an online poll and it was part of a psychology experiment. The methodology was to assemble a cohort of (I believe) more than 200 sexually active women in the age range 21 to 40, and use different-sized dildoes to measure arousal. Arousal measurement was done by measuring vaginal wetness and filming pupil dilation. No verbal cues at all While not the last word, it was a fair attempt at answering the question, and I think that most would agree that the results seem reasonable. Longer than average is preferred, as well as thicker than average, but not to extreme in either case.
This experiment does not prove that women who have average-sized partners are unsatisfied with their sex lives, but it leads us towards the conclusion that those women would, in most cases, be more satisifed if their partners were larger.