My buddies over at the FootJoy blog have a very informative post about shoe sizing. Apparently, most of us (80%) are walking around in the wrong size shoes. They show an easy way to tell if your shoes are the right size by using your thumb to measure.
The width of your thumb, about 5/8″, should fit perfectly between the top eyelets of the FJ golf shoe. If the eyelets are not visible under your thumb, and are pulled closely together, the shoe may be too wide. If the eyelets are too far apart, the shoe may be too narrow. See the image below for an illustration.