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.
Hush your mouth with using old make-up brushes!!! If they're no good for the make-up they're supposed to be used for there's ALWAYS other makeup they work with!!! Unless, of course, you're thinking that I should use make-up on some of the mini's.....
the only rule with which brush you use to dry brush with is : don't use a brush you want to do delicate work with afterwards - other than that use a brush the appropriate size for the area you are dry brushing
~The ravings of a single mad Goblin is bad enough, but such a power-hungry, malice-filled creature as Mortis can never hope to be understood~
Yeah Mortis makes the important point: dry-brushing knackers brushes. I tend to use older brushes of various sizes depending on how big an area/how fine the detail is. My favourite supply of brushes is EM4. Their sythetic brushes are pretty good but still very cheap, and go down to 0/10 which is pretty damn small. The sable brushes are way nicer and a bit more expensive but still a lot cheaper than most places. Oh and the handles are big and triangular which I find much more comfortable.
I found it easier to learn drybrushing using a larger brush -a citadel one in fact. And that it seemed easier to learn using the flat brushes. Once you "have the feel" of drybrushing it can be done with smaller brushes, so I often use the much smaller foundry drybrushes now for certain jobs.
As a general rule, have specific brushes for drybrushing. Try a bigger brush.
I was planning to use them for drybrushing but have ended up using them for blocking in large areas. Not only do they give a smoother finish but really save a lot of time and seem to use less paint. Kicking myself for not trying this years ago