What do the letters in the size mean?

The letters represent width.

For ladies, we stock B, C, M, W
B and M represent a medium or more narrow width
C and W represent a wider width

For men, we carry D, EE, EEE, EW, W
D represents a more narrow fit
EE, EW and W stand for a wider fit
Occasionally we stock EEE, which is very wide
Just about all our men's footwear is supplied in one of the wider sizes