The proper wire size for an electric range is #8 AWG for a 40-amp breaker or fuse. A.W.G. refers to the American wire gauge size, which is a standard used in North America.
The proper wire size corresponds to amperage, and using the proper gauge means that the wires will not overheat. For a 50-amp breaker or fuse, the corresponding wire gauge will be 6 AWG. The material used for the wire itself also makes a difference. The figures given assume copper wire. For aluminum wire, the size should go down one number in gauge size.
It is very important to use the correct gauge wire. Otherwise, in the event of a short circuit, the wire can overheat and burn, potentially starting a fire.