How long is the recommended length of thermowell compared to pipe diameter?
Thermowell length is recommended to have thermowell tip lies on the middle third of pipe. This is intended to have more accurate measurement of process temperature. See below picture for detail.
In large pipe size, the above method is probably not applicable, due to longer thermowell which subject to break due to process stream. In any case, the thermowell size shall fulfill the requirement of ASME PTC 19.3.