I have some 24 and 26 TMCs that I have tied up, but they are 1x. The size refers to the distance of the hook point to the shank (the depth of the bend) if I am not mistaken. This just reduces the amount of hook in the water and (in my mind) keep picky fish happy. For standard dries I don't go smaller than 20 myself.
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DS Fly Fishing
The DSFlyfishing Blog Cuz' Ya Gotta Fish.