6 inch thermocouples are ideal and interchangeable with most kilns.
Thermocouples are made of two types of wire. The temperature is read from the thermocouple probe's tip that is inside the kiln and where the two different wires meet. All thermocouples are designed the same way and are interchangeable between kilns. The length of the thermocouple will be determined by the thickness of your kiln walls.