I'm just a little north of 7 1/4" (real, from the top, properly measured inches, not internet inches), and I've been with a few girls that felt a little too long for, but for the large majority it's felt like the perfect range. My current girlfriend's said that after I get deep enough (past the posterior fornix, though she didn't use those words..) she doesn't have much sensitivity past there. For us, an extra inch wouldn't make a difference for her, and for me would just leave more of me out in the cold air for longer, so it'd be worse.
I also find that with my up-curve, at my length I'm usually stroking straight from the a-spot to the g-spot over and over, so if anything another inch would throw off my rhythm there.
Of course, my dick is the only one I've ever had, so maybe I just think it's perfect because I have figured out how to use it. If I was around 4" I'd probably love that too.. I think it could be awesome to get my body in nice and close and really work the clit with the base of my dick while fucking. At 5" I can see how I could push in to that g-spot more than I possibly can now. 6" would be versatile and work well in almost every position, look just as good, but with way lower odds of going in on a bad angle and smashing the cervix. 8"+ would be cool for really working the CDS, so for the women who love that they'd appreciate it even more.
I think trying to just say "x is better than y" is a little misguided. If you're just going by what appeals the most broadly to the most people, then generally average is better (that's usually why it's the average in the first place.) You could also look at it like a supply and demand situation, so maybe more people prefer X to Y, but if there are way fewer Ys out there then those people who want Y over X will value Y even more.