/ic/ - Artwork/Critique

Strange question but directed at digital illustrators/fine arts types, what would you say are the best brush sizes to use for a piece? I have this constant issue where I can't find a good combination of sizes so I have a small brush for fine details and a thicker brush for major linework without it looking too thick, I can't seem to get that balance and I figure this would be the best place to ask so any help, online tutorials, images, anything to help me with this issue so I can get closer to pic related would be absolutely invaluable to me and much appreciated.