The measurement of a wood chisel refers to the width of the blade’s cutting edge.
There are far too many sizes of chisel to mention them all, but wood chisels commonly range from 6mm to 50mm (1/4″ to 2″), typically rising in 2mm (5/64″) increments. However, there are many other sizes of chisel available in both metric and imperial denominations.
No individual size of chisel is designed for a particular task. However, you should always try to use a chisel that will closest match the width of cut you need to make. By doing this, your cuts will be clean and accurate.