Boot and shoe manufacturers use mainly a simple guideline of sizes to use when preparing for a line of shoes or boots. You will see the following:
N-Narrow, M-Med, W-Wide
B-Narrow, D-Medium, E-Wide or EE extra wide
There are a few times you may come across more widely used sizes where A, B, C, D, E, EE, EEE, EEEE, etc are used, so that a manufacturer is more able to fit the entire population.