Every year we get two different black stones that show up with the warmer weather. One is small--like a sixteen. But the other is big. I like it on a 2XL #14 or a standard 12. That's probably over sizing it, but the trout don't seem to care.
When you're fishing the stonefly, be sure to twicha nd skitter it. Work it in all the same places you would a streamer. Fish the wood piles and the bubble lines. Drop a nymph off this beefy critter. It makes a fine bobber.
I like fishing the stone best upstream as a single fly. Really focusing on the secondary and tertiary bubble line behind logs.
However you use it, it can be a great way to get your first dry fly fish of the year.