There might not be an exact consistency in the size rating, but it can
be used as a general guide when asking for a certain brush size. Each
companies #0000 might be a hair or two different in size, but it will
be kind of close... oop's, no pun intended.
I have some that are as small as (and marked) 30/0 and 0000. I found
them at an art store, well my wife did, she paints oil on canvas. If I
have to paint anything smaller than that, it's time to dig out the
toothpicks and start whittling LOL.
Another note regarding brush sizes. It also depends on the shape and
taper of the bristles, meaning what type of brush style. There's flat,
round, short round, script, filbert, etc...