This is depending on the accuracy you expect to gain from the calibration. If sensors are having a diameter of more than 9 mm the heat loss up through the sensor is becoming very high. This means that the calibrator should somehow be able to compensate for the actual heat loss. This can be secured by using a calibrator with a very long heat zone or by a calibrator that performs almost the same temperature at all heights. The other solution is choosing a RTC calibrator with individual heat zones.