The postal dose audit using radio-photoluminescence glass dosimeters was begun in November 2007 in order to improve the quality of radiotherapy in Japan. However, the irradiation conditions are now limited to the reference conditions which are 10×10 cm(2) field and 10 cm depth. The application of the glass dosimeters to non-reference conditions is strongly desired. This study dealt with the field-size dependence of the glass dosimeter outputs in the 6 MV photon beams of a medical linear accelerator (Varian Clinac21EX). We irradiated glass dosimeters with square field sizes of 5, 7, 10, 13, 16, 20, 23 and 25 cm side lengths at the 10 cm depth of the water equivalent phantom (SSD=90 cm). The outputs were compared with ionization chamber outputs. The ratio of the glass dosimeter outputs to the absorbed dose to water obtained with the ionization chamber increased approximately 1.5% between 5×5 cm(2) and 25×25 cm(2). We have to consider this field-size dependence when we apply the glass dosimeters to non-reference conditions.