If you're painting or varnishing woodwork, choose a small brush from more precise control, A 1.5-inch wide brush is ideal for most woodwork jobs. But, if you're painting trim that's wider than 3-inches, a little wider brush, up to 2.5-inches, would be a better choice. If you're painting walls you'll use a paintbrush primarily for cutting in. Pros sometimes use 3-or 4-inch wide brushes for this, but we think a 2.5-inch wide brush is ideal for most DIYers.