I rarely fuck them, only suck them, so my preference eliminates the first variable. So while a big cock is nice to fuck, it doesn't really come into play when sucking. But a small cock is not as exciting to suck, it gets lost in my mouth. The size of the head and the overall diameter is probably more important to me than the length. I can't fit more than 7" down my throat, but the sensation of a huge mushroom head between my lips is definitely more exciting than a small wienie.