i usually use a size 00 or 000 size brush for the outline. One thing ive learned about outlining with a brush is that it helps to have a more fluid paint. I used Golden fluid acylics, because they are still opaque enough to cover in one line, and they are much more fluid than regular acrylics. I also use a certain product ( some sort of gel medium) that i mix with regular acrylics, that makes them more fluid, and stay wet longer if your blending. it works well, but not quite as good as the golden fluid acrylics...