Each girl is different.. I like to think of it like this.. Just my thoughts, from reading, conversations, and experience...
~ 5% of women want below average - they are very small.
~ 35% of women want the average range 5-7 in length and 4 1/2 - 5 1/2 in girth
~ 55% of women want above average range 7-8 in length and 5 1/2 - 6 1/2 in girth
~ 5% want as big as they can get - welcome to size queens and a good portion of the women on this site.
Once you start hitting the 7 1/2 X 6 area you will find women that you will bottom out on (depending on position) or may need some extra time getting things started. In the end just like every guy is different so is every woman. They just have a neat ability to expand given time, and patience.
From my experience woman want to be able to think to themselves wow big which is tempered by is he going to hurt me which is relative to there size and past experience.
Also you don't hear a lot of women saying I want a huge penis because that can be misconstrued as I have a gigantic vagina and need a gigantic penis to satisfy me (most women want to be thought of as tight). For some unknown reason it has been deemed acceptable for women to say I want 8 inches with very little negativity coming back at her. However, if you see a woman say I want 9 or 10+ look at the comments that follow, she normally will take some shit, from insecure men, that forget everyone is different.