To determine sock size, stand on a piece of paper and trace the foot with a pencil, pen or marker. Trace the other foot using the same method. Measure the long side of the tracing to determine length and the short side of the tracing to determine width.
Experts at About.com recommend, "After you've come up with your two biggest numbers (length and width) you'll want to subtract 3/16 of an inch from each of them." This is to account for space made between the foot and the pen or pencil. The final measurements are the size of the feet. Consult a sock or shoe chart to determine the appropriate size.