The easiest way, if you are replacing an existing item is to look at the body of the part to see if the size is stated.
To measure, you need the measure the widest thread on the male thread, see here for more information
Manufactured from 316 stainless steel tube, this is a Male BSPT x Weld nipple. Threads are to ISO 7-1 (Taper/parallel). These fittings are suitable for use in the following types of installations sanitary, heating and gas installations, cooling and air conditioning, sprinkler and gas fire extinguishing systems, construction of industrial plants, machine construction, petrol stations, paper production machines, printing machines, commercial vehicle construction. Maximum work pressure is 20 bar upto 260°C.
The easiest way, if you are replacing an existing item is to look at the body of the part to see if the size is stated.
To measure, you need the measure the widest thread on the male thread, see here for more information