This solid state thermostat is accurate to 1/10 of a degree! Maximum rating is 1 Amp (pilot duty only). The temperature sensor is 2" long by 5/16" thick and on a 24 inch cord.
The thermostat probe should fit completely inside the thermal well. It is OK to replace with a smaller thermostat probe, but not a larger one. If it is going to protrude outside of the thermal well it should be insulated. An easy way to insulate the back of the thermal probe is to cut a slit in a Styrofoam cup. Then fill with spray foam insulation and cover the rear of the hot tub thermostat probe.
If you are replacing a standard mechanical thermostat, some custom wiring may be required.