I've seen this passed around through text messages and a lot of girls myspace:
8 is pain
7 is heaven
6 is perfect!
5 is okay...
4 is meh
3...
2..
1 fuck it, use your tongue!
Of course this is supposed to be funny, but I truely do think the "ideal" size for girls as a whole (not just size queens or girls who cannot handle more than 7, because they both DO exist) would be in the 7 range. It caters to both sides.
I guess one comparison I could make to it is my hieght. I am 6'1 or 2, I'd like to think I have a fairly "ideal" height in the sense that I can cater to women of most all heights. I can get with both short and tall girls without it looking strange. I am tall enough to be considered "tall" without dwarfing (and discouraging) short girls, plus I won't leave a taller girl to look like an amazon woman next to me.
I wish my cock was in inch bigger! I would donate an inch of my overall height to my penis, maybe. 7.5 (with proportionate girth) sounds great, God.