The apparent fracture toughness K_c is an important material property that characterizes the ability to resist crack initiation and propagation in the polymer bonded explosive （PBX）， investigating the size effect of apparent fracture toughness K_c is of great significance for predicting the crack initiation and failure behavior of PBX at different scales. This study examines the size effect characteristics of the apparent fracture toughness K_c of PBX surrogates by cracked straight through Brazilian disk （CSTBD） tests. Fracture tests were conducted on CSTBD specimens to investigate the influence of pre-existing crack lengths （2， 4， 6， 8， 10 mm） on the apparent fracture toughness K_c. The size effect of the apparent fracture toughness K_c of PBX surrogates was explained using the traditional maximum tangential stress （MTS） criterion and the modified maximum tangential stress （MMTS） criterion， which considers the higher-order coefficients of Williams series and the length estimation model of fracture process zone （FPZ）. Results show that， as the crack length increases， K_c increases from 0.139 MPa·m^0.5 to 0.251 MPa·m^0.5， and K_c tends to stabilize with the increase in crack length. With the use of the A₃ term-corrected FPZ length estimation model， the modified maximum tangential stress （MMTS） criterion can effectively explain the size effect of the apparent fracture toughness K_c of PBX surrogates compared to the traditional maximum tangential stress （MTS） criterion.