This is from the AE site:
"Allen Edmonds shoes come in lengths from 6-16. Each half size increases by approximately one-sixth of an inch and each full size increases by one-third of an inch. The widths of Allen Edmonds shoes range from AAA, the narrowest width, to EEE, the widest width, with a D width considered medium. Technically speaking, each width increases by approximately three-sixteenths of an inch in total girth."
I knew I read this once and guessed it was in the All about Last page.