I have never tied anything below a #18, but have bought some. It always amazes me when I catch a fish on something that I can barely see, especially when the 6X tippet looks like a rigging cable when you compare it to the fly. I

bought a bunch of #20 and #22 BWO for the Yakima one day, and opened my box while standing on the Umptanum foot bridge to compare them to a bug sitting on the cable. They flew away in a gob, and I was stuck trimming on my #16 flies to match the hatch.

I mostly fly fish lakes, so I can usually get by with flies no smaller than #18, and that is for one specific hatch on one specific lake, which I have only hit twice in my life. The rest of the time #4 through #14 are fine.