Well, we're not going to knock what others like or think is possible... but I can say from my wife's experience that girth trumps length
She's had up to 10" so far, and at first sight she thought there was no way she could take that, but she definitely did and loved it!
Have to say that really long big cocks do look amazing sliding into her pussy, for both of us, but for actual pleasure length isn't always the best for her.
So she'll tell you that 6-8 long, and
THICK is typically what does it for her. Any longer is usually a less girthy cock, sometimes doesn't really stay rock hard, and too much longer bottoms out and can sometimes be painful.