For the purposes of my patterns and lasts, the length of the foot is the determining factor when deciding which to use or purchase, as commercial shoe sizes are so variable.
To determine the length of your foot, stand on a piece of paper with your heels against the wall. Have someone make a mark at the end of your longest toe, then measure from wall to that mark. For width, simply let me know if your foot is narrow, medium or wide.
Note that you don’t need to add extra toe room, as your foot is actually pushed away from the wall usually 3/8″ by the protrusion of your heel; this extra space will end up providing the toe room.