I would say if you can find any of the Sables, those last a bit longer than a regular brush but it also depends how good you clean them and it takes just a bit of maintenance with some brush cleaning solution or even rubbing alcohol to save the longevity of the life of the brush.
For size, I use anywhere from 00 to 000 rounds, there's also some very very fine brushes if you can find pinstripe brushes, the smaller you get the steadier your hand has to be IMO. I have VERY steady hands, so it's not a problem since I can hold a position without moving for the amount of time that I need to add detail to the iris. I would say that's at least one of the greater factors that helps out quite a bit whenever doing eyes. That and your vision, I know it sounds too fundamental and of course you have to "look" at sculpts all day while painting but I don't have to strain to look at something that tiny where as I've talked to other painters and they are just bugging out and have to step back and take breaks, etc...
A lot more goes into than a lot of people realize, definitely have to keep a pace and remain patient.
Ski
Yeah, I have no illusions of the ridiculously perfect eyes you and some of the others are able to achieve. But just fairly clean circles will be good enough for me at this stage of my painting
Luckily I do have good eyesight and a fairly steady hand... but when you can see that stray bristle f'n up your ____ you know it's time to move on to better brushes than the $8 30 pack
Wow, this last photo group shot looks killer!
Finally the Fellowship's journey has come to an end! 
One last set to show how well these little guys fit in.
I only include Frodo and Sam to show how seamlessly they fit in. Merry is a much better sculpt in terms of likeness, Pip is decent, but not great.
I recommend you send your mispainted stuff to ski so he can do it right.
............i usually do my own painting (though my eye paints will never match his talent at this) but i just had to have at least one ski masterpiece in my collection...............
my ski-painted x-wing luke is 4-ev-r..............
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