Small ones seem to work better. They're easier to suck and produce just as much cum. The guy attached to that small penis is much less likely to be a cocky asshole whereas the guys with a big dick are often all dick. Guys with small cocks tend to keep the bush trimmed to make it look a bit bigger an it's appreciated by us cocksuckers. So if I was at some sort of sex party and there were a few men, naked and hard looking to cum, I'd pick the smallest one first.