The Right Fly

I believe the most critical property of a trout dry fly is the size of the fly.  For example, one morning on the trico hatch I decided to fish a size 14 fly instead of the size 24 Trico I usually fish.  Although the two flies looked much alike I caught only 4 trout on the size 14 as compared to 20 or so I usually caught on the size 24 during the hatch.