Hi,
I regularly fish with flies of sizes 20 - 24 and smaller...
The most common ones i use in size 24 for example are CDC F fly & Griffiths Gnat, I also have very small blue winged olives, Midge patterns, caddis... etc.
I also have some bead head nymphs that go down even smaller than 24 .. So small i cannot see to tie them on!
I find that quite often small Brownies will grab the tiny flies when other offerings are being ignored? Most often in dry form too?
I have also used them on small still water when fussy feeding habits are impacting my catch rates.... Its surprising that something so small grabs so much attention but it works!