How Small a Trout Fly Do I Need?

Fly Fishing Tip: How small of a Trout Fly do I need?
To me it depends upon what the trout are feeding upon. For example, if they are feeding on natural pseudocleon or tricos which are size 24 on the streams I fish, then I use a size 24 fly. Late in the season when the trout are feeding on a variety of terrestrials I’ll go to a size 20 Ant if they will not take my larger patterns.