I like to compare with the sizes on screen. I like it when I am bigger than the porn guy's dick (which only happens with porn with real life averages). And I don't mind when the dick is bigger than mine.
However, other than the rare novelty of watching a huge dick, I usually don't like to watch huge dick porn, like mandingo. Any more than 8 inches, and its all about the dick it seems like. I like my porn to be about a chick getting fucked, I'm not so much into worshipping some guy with a bigger dick than me...
However, I do like to see a big dick stuffed in a nice young ass.