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
Description
Manufactured from 316 stainless steel tube, this is a Female BSPP hexagon blanking cap. Threads are to ISO 7-1 (Taper/parallel). Hexagon caps 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 up to 260°C.