It\'s the point, not the brush.
The difference isn\'t in the size of the brush, it\'s the size of the point. That, and what it\'s made of. A Kolinsky Sable round or spotter, size 3/0 will work better than a Golden Taklon 20/0.
My favorite brush of this type is a Windsor & Newton Series 7 miniature. Being a miniature, it has shorter bristles than a round. This type of brush is also called a spotter. The point on this baby is as sharp as many smaller brushes, but it has enough hair to hold a workable amount of paint.
The flow qualities of your paint have a big impact, too. Learn to thin your paints, and use additives if you need to.
The last component of good detail painting is brush control. When I first started painting, I tried to make up for lack of brush control by using tiny brushes. So I have a collection of truely nasty teeny-tiny brush that no longer get any use.
A well-stocked gaming store that caters to painters should have some really good brushes. If you can\'t find what you want there, try an art store. Last of all, go online. Jerry\'s Artorama has a huge sellection of brushes. http://www.jerrysartarama.com
So: get a good quality brush. Thin your paint. And practice, practice, practice. That will do a lot more for you than a teeny-tiny brush.