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
2 Piece Full Bore Stainless Steel Ball Valve, Body and Ball made from CF8M stainless steel with the seats and seals manufactured from PTFE. Working Temperature -20°C to +200°C. 140 bar Rated. Anti blowout stem and Lockable Lever as standard. 3.1b Certification is available on request. Option of BSPP or NPT female threaded ends. Albion type 912P/912NPT