The OP is asking about pedal width, not crank width, but I'll chime in with my method for measuring the crank Q-factor. This is easiest with a caliper but a ruler works okay as well:
Line one crank up with the seat tube and measure from the outside face of the crank to the tube, then do the same with the other crank, then measure the diameter of the tube. Add all three numbers together and there you go. With three separate measurements there is a little more chance for error, but I can usually repeat the measurement within 1-2mm so close enough.