What is the best size paintbrush?

This would depend on what you are painting as paint brushes come in various shapes and sizes.

I would say for detailing you should a small brush, the exact size will depend on what you are painting. If for example you are painting a face, I would be inclined to use a 0 or a 00 brush which are the larger of the small brushes and if painting eyes a 00000 brush would be preferable as this is the smallest.

For larger areas requiring paint, I would suggest finding a size that you are comfortable with, starting with a number 2 or 3. A lot of model making is trial and error and there is no right or wrong. Find what works for you.

Always wash out paint brushes after use and repoint, especially the smaller brushes.