Paint Brushe sizes

I wrote Windsor-Newton last year and asked them a similar question. Here is part of their response:

"Different manufacturers, producing product in different countries can use completely different numbers to indicate a brushes size.

Basically uou just have to look at them to see which is smaller.

I was under the impression 5/0 was 00000... but writting that isnt realistic. either way; if their local; pick whatever looks good for what you need. past 3/0 or so; they seem about the same. I mean what can 5 bristles fit into that 10 cant. their all sooo thin