Based on the fracture mechanics concept, effects of specimen geometry on the tensile and compressive strength of magnetite-based heavyweight fly ash-based geopolymer concrete have been investigated both experimentally and analytically. In the experimental program, three mix designs have been prepared containing 0, 50, and 100% magnetite heavyweight aggregate (HWA) and were subjected to the fresh and 28-day mechanical properties tests. For that, 100 and 150mm cubic specimen, and 100×200 and 150×300mm cylindrical specimen for compressive strength measurement, cylindrical specimen with the same dimension as those for the compressive strength, for splitting tensile strength test, and 100×100×400mm prism specimen for flexural strength were prepared. In the analytical section, by applying modified size effect law (MSEL) model and justifying its parameters for the heavyweight geopolymer concrete (HWGC), correlations between the compressive and tensile strengths of HWGC with the specimen geometry have been developed. In addition, correlation between the modulus of rupture (MOR) and compressive strength of HWGC was also established. The obtained results denoted the crucial role of the HWA in the fresh and mechanical properties of fly ash-based HWGC, alteration of the mechanical properties by varying the specimen geometry, and correlation between the MOR and compressive strength of fly ash-based HWGC.