My experience with them suggests assorted sizes from about 4 to 10 or 12 to cover trout and steelhead both. I don't tie, but fish them with assorted head colors from dark red to paler colors, to match the changing colors of the naturals. I also fish them with bunny tails and more sparsely tied, depending on what's working.
If you swing them under spawning salmon, hang on
Hope I helped...-m