Most flat washers are shown with a single sizing option: Diameter. This indicates the nominal inner diameter of the washer AND the bolt diameter this washer is designed to work with. Select the same size washer diameter as the bolt diameter.
An exception would be Fender Washers. By design, these allow you to select from two size options (i.e. 1/4 x 3/4). The first indicates the nominal inner diameter. This is the bolt diameter it works with. The second indicates the outer diameter of the washer itself.