Also from Gary LaFontaine's book "The Dry Fly", he talks about picking the size based upon water type/speed. Whereas the most common sizes fished with attractor dries like Royal Wulffs, Trudes, Humpies, etc. is usually about #12-14, he claims these are some of the LESS useful sizes.

In slow/flat water, he recommends #16-20, and in heavy, fast, broken water he likes #4-10. You have to give the trout a big enough food item to pull them up from the bottom in deep, fast water. And conversely, fish are suspicious of overly large items in flat water. I guess one would use the #12-14 in gentle riffles.

He has an interesting strategy based upon his underwater diving research. There were bigger trout in the pools that virtually never rose during hatches, instead staying deep and feeding only upon nymphs. Some of these trout could be enticed to the surface, however, by blind fishing really big attractor type dries in the heavy water at the pool heads, in the faster water. If trout are rising off the current edges in 2 1/2' of water, then fish farther into the current over 4 feet of water. If they are rising 30 feet down from the pool head, then fish up above them in the extreme head of the pool. Interesting stuff I think.