How distinguish paint brush size specifications？

The most common paint brush specifications on the market are: 1 inch (25mm), 2 inch Paint Brush (50mm), 3 inch (75mm), 4 inch (100mm), 5 inch (125mm). In the process of using 5-inch paint brush is less used, the more commonly used are 3-inch and 4-inch paintbrush, 1-inch paintbrush mainly handle the general corner of the paint brushing.