I've had them all over the board and done a considerable amount of "comparison shopping" before I bought one.
I would have to say that 8.5" AND some nice fat, juicy girth is optimal. I've got 9" at home, and I would prefer him to take an inch off and add it to the girth. He's not 'pencil-dicked' and I am certainly not 'roomy,' but sometimes I think that the extra .5"-1" would be better used on the sides. This is, of course, my lifelong partner.
Now if it were someone that I was having a short on non-frequent fling with, then HELL YEAH I love a FAT 10"!!! Anything more for me is only good for a bladder infection.