Originally Posted by Eric
The size of the fly is determined mostly by what the fish are likely feeding on. If the trout are feeding on insects and insect larvae, then smaller hooks, generally.
Until a trout is over 12" it eats bugs. Over that size it eats bugs when plentiful and other items like small fish or crayfish. Large trout that mainly eat large items usually don't feed until dark (that's how they get large).
Most of what trout eat is 5/8 of an inch and brown. And in a normal current of 1ft/sec a trout will take something into its mouth every 6 seconds. So much of the time you could use size 12 or 14 as the largest - unless you are fishing a minnow imitation, in which case you will catch few but they will be larger.
I would suggest whenever you see bugs or any surface activity you use size 14 or 16 hooks either above or below the surface. On the Farmington (Conn.) at this time of year, on the Deerfield, and on the Grand in Ontario I would even go down to size 18.