I've only got a few flies in my box that are smaller than size 18. They're my go to flies when the fish won't take anything bigger - which is rarely. Then again I dont match the hatch much, I fish Royal Wulffs, Irresistables, Adams, and EHC almost exclusively when dry fly fishing. In the warmer summer months I use a big grass hopper as a point fly, with a smaller dry on a dropper. If I'm fishing a three fly rig, I might put an 18-20 little black gnat on the end...but that's rare.

I gotta say, after tying a bunch of them little flies, tying a Wooly Bugger is a piece of cake