SS = AAAA = "extra slim" or "quad", the narrowest size generally available. Even these widths are increasingly rare.
S = AAA= "slim" or "triple"
AA = N = "narrow" or "double A"
M = B = "medium", the most common or "average" width for that size.
W = C or D = "wide"
WW = EE or EEE = "extra wide"
WWW = EEEE = "triple wide'.