I think every woman is different, and, having been a dirty little whore for most of my life before I got married I have to say...
I have a tilted uterus/cervix that makes anything longer than, say, 6-7 inches, especially doggy style HURT LIKE HELL.
Also, I've been with guys with large penises that seem intent on bruising the shit out of my genitals because they think girls like that. Sorry, but having to wear ice packs in my panties is just not my thing.
I've been with men that are average/small that are great in bed, and men that are average/small that are terrible in bed.
I really don't think it has anything to do with size for me in terms of my enjoyment because all of my sexual experiences have been so wildly different from one another that just penis size is only one small factor.
However, everything else being equal, I'd have to say that average length, decent girth is what I prefer. Just... I can't do the skinny pencil dick. Actually, I'm lying. I can. One of my favorite vibrators is no longer than 4 inches and the girth of a magic marker.
Actually it's my husband's vibrating Gilette Sensor razor handle. I am a BAD BAD girl.
Snooz