hm. i'm big--quite big, both for my age and, well, for today's world. although i go for the mental click, if you ask me what traits i would look for in a dominant physically i wouldn't go for the skinny type; i like knowing that pinning me down wouldn't be hard for a Dom, even though i would be willingly submitting to him. that, however, isn't a general rule. although i'm pretty strong, when i'm in my sub mode it seems like only the cheek is there and the strength goes away *grins*.
bottom line, though, i agree with what Oz said. Dominance is about personality--small or big, tall or short, if they've got it, they've got it.