To get a better idea of the size of the brush you should use for a particular job, here's an estimate of which brush size works best on certain areas.
1" - 2" Brush
This brush size is ideal for painting windows, shutters and items such as frames and mouldings. A similarly small angular brush is perfect for clean neat edgings, sashes and narrow trim work. These sizes are also recommended for touch up and smaller jobs such as toys, tools, furniture and hard to reach places.
This size works well on larger flat surfaces such as cabinets, cupboards, flush doors, beams, fencing, stair steps and shelves.
This size should be saved for very large flat areas such as walls, ceilings, panelling and floors.
Always use a brush which is comfortable to work with, never use one which is wider than the surface to be painted.