This document specifies the determination of the size of gamma radiographic sources of 0,5 mm or greater, made from the radionuclides Iridium 192, Ytterbium 169, Selenium 75 or Cobalt 60, by a method of radiography with X-rays. The source size of a gamma radiography source is an important factor which affects the image quality of gamma ray images. The source size is determined with an accuracy of ± 10 % but typically not better than ± 0,1 mm. The source size is provided by the manufacturer as the mechanical dimension of the source insert. A measurement may be required if the manufacturing process is validated or monitored after implementation of the source into the holder. This document can be used for other radionuclides after validation. The standard test method ASTM E 1114 provides further information on the measurement of the Ir-192 source size, the characterization of the source shape, and its correct assembly and packaging.