I'm sure if you check back through the older posts on this sub-forum, you'll find a post or two about a fly tied on a Mustad size 40! Supposedly there was just one small production run of those back in the 70's, and every now and then, some will turn up.

I've got some TMC 101 size 24's, and I've tied a couple flies with those. A size 32 (or smaller) is just pure insanity!