It depends on a lot of things, really. There's a convergence of factors that all have an impact on what I prefer. The things I consider when determining what I "prefer"--a guy's gotta bring something other than just a big dick to the party.
General looks, kink compatibility, cock aesthetics/endowment. Most important to me is kink compatibility. A guy can be average looking, have an average cock, but if we share a mental connection, that will carry things far. I'm definitely more wild than mild, so vanilla guys won't hold my attention, no matter what they're packing. In regards to endowment/cock aesthetics there's several things I really like that aren't so much about "endowment". I prefer girth to length, love a big mushroom head, love heavy pre-cummers/cummers, love veiny cocks. That said, I've been presented with some absolutely gorgeous big dicks that I loved playing with--their faces weren't a factor cuz I was too busy looking at their cock, and even if they're quiet and a lazy lay, if they can keep it hard while I ride them, I can enjoy their size. As long as a guy can hit my prostate, they can give me pleasure.
That said, my first hands-free orgasm where I came with zero cock stimulation came from a guy packing 11", and knew how to use it. But I think that was more because we were very sexually compatible.
If I was making a wish list, I'd take a guy with average endowment but was very Dominant, kinky, and vocal over a quiet, hung guy.