Never use a brush smaller than your head

Once piece of oil painting conventional wisdom goes along the lines of "find a brush size you are comfortable with, then get a larger brush and use that." The idea is that larger brushes help you to paint in large abstract masses rather than getting stuck on fiddly little details.

For my part, I tried using larger brushes than I felt comfortable with for some time. I felt like I was trying to paint with a cat. After a couple of months, it wasn't any better. So I got some smaller brushes, and finally felt like I could control the paint. Although it's important to understand the need to see the big picture and work from larger to smaller, my advice is to not take that principle to an extreme. If a big brush is helpful, then use that. If smaller brushes let you get the job done more effectively, then use those. Often, it's a good idea to work from larger brushes to smaller ones as the painting progresses, but don't drive yourself nuts with a brush you can't control.