I've just had a look in my jam jar of brushes and my rigger brush size is no.2. This produces very fine, pencil line thicknesses of paint.I can't advise on the other sizes, as I've never used them. If you do get one the secret is to keep them well loaded with paint. They look as though they won't produce thin lines, but they do......Trust me, I'm a Doctor!!
Oops, sorry Steve, John and I's posts crossed......