It's not so much the size of brush as the texture of the bristles, I think.
For dry-brushing you need something relatively stiff, so it will flick over the details better. But even that isn't neccessary, really.
I am not being helpful.
All you need to do is practice. Once you have the technique down, you can find out for yourself which brush is best, be it for being the cheapest to replace, or just having the right.. "Feel"
You could always use old make-up brushes e_e
Mankinds first mistake; Questioning why those around him, are dying.