Go Big or Go Home
Other artists often ask me how I paint so loosely. And I tell them my secret; use big brushes! Really. That's it.
But that's not what they want to hear; they say "I've tried that". Well, it is difficult. Forcing yourself to use large brushes means you must edit what you are painting; you have to omit small details in favor of larger statements that define the most important shapes, angles, and proportions. That's the real challenge.
That's the next question. The short answer is, "the biggest you can deal with." For the long answer I've diagrammed the brushes I used on two recent paintings. To understand the proportion of the brush sizes, know that these paintings are 36"x36", and that my brushes vary in width from a #12 (1" wide) down to a #4 (1/4" wide). The brushes I use most often are #10 (3/4" wide) and #8 (5/8" wide). By the way, the width of a brush is measured at the ferrule, the metal part, not at the bristles, which can splay.
Natural vs. Acrylic
You might notice that I have two types of brushes; the green handled ones are natural bristle brushes (Grand Prix by Silver Brush) and the blue handles are acrylic bristle brushes (Bristlon by Silver Brush). I like the natural ones for large brushes; they hold a lot of paint and give me a soft edge to my stroke. I like the acrylic for small brushes. They have stiffer bristles with lots of spring which gives me a lot of control and a sharp edge to my stroke. (See this blog post to learn more about how I select and care for my brushes.)
Step Away From Those Small Brushes
I do have to work on using big brushes. If I find my self reaching for my #4 too often I know I'm getting stuck in fussy details which is not what I want for my paintings. So I stand back to evaluate my canvas and determinedly pick up a bigger brush.
I hope this has been helpful. I'd love to hear your comments!