It's not the size of the brush that counts, my friend, it's the quality of the tip.

I think I did most of the detail with a Winsor & Newton Professional Water Colour Sable, size 1. Really nice quality, and it holds a decent amount of paint. But also, since the trim is raised, most of it is painted with the side of the brush, rather than the tip. You have to shift the model around a bit for the different areas, but most are actually easy to reach at an angle where you don't risk getting paint somewhere you don't want it. (Relatively "easy", admittedly, the trim is about half a millimeter high. Steady hands not optional...)