Use AWG (American Wire Gauge) standards to specify the size of your wire. AWG rating, assuming that the component wires of the thermocouple wire are equal in diameter, relates to the diameter of each individual wire in a formulaic manner. Wire diameter, in inches, equals 0.005*(92^((36-AWG)/39)). Using this equation, you can calculate the wire gauge of your thermocouple wire, which both the electronic and electrical fields recognize.
Size the thermocouple wire, alternatively, by the diameter of the conductors. Each of the two conductors will be the same diameter. Check the manufacturer's specifications to determine what this diameter is, or use a tool such as a caliper to size the diameters of your conductors.
Size your thermocouple wire by the cross-sectional area of one of your conductor wires, using this formula: Area = 3.14*(radius^2).