To analyze the effect of specimen size on the compressive strength of ground granulated blast furnace slag-fly ash (GGBFS-FA) stabilized large-size macadam, the 200 mm × 200 mm × 200 mm cubic specimen and 3 d compressive strength were designated as a reference specimen and strength in this study, and numerical simulations of compressive strength were performed by using a discrete element method for GGBFS-FA stabilized large-size macadam specimens with a maximum particle size (d_max) of 73 mm and five side lengths of 2.0d_max, 2.5d_max, 3.0d_max, 3.5d_max, and 4.0d_max. The size effect mechanism on the mixture’s compressive strength was revealed from the perspective of energy. By comparing the applicability of the improved Baz?ant size effect theory model and the exponential size effect empirical model to the size effect on the mixture compressive strength, the critical size of the specimen and the strength conversion factor were proposed. The results suggest that the compressive strength of GGBFS-FA stabilized large-size macadam shows a significant size effect, and it decreases with the increase of specimen size. When the specimen size exceeds 3.0d_max, a decrease in the total energy and elastic strain energy gradually stabilizes, resulting in a weakening of the size effect on the compressive strength. Compared with the improved Baz?ant size effect theory model, the exponential size effect model is more suitable for the size effect on the mixture compressive strength. It is recommended that the critical size of the compressive strength specimen for GGBFS-FA stabilized large-size macadam be set at 256 mm, and the compressive strength of the specimen is 95% that of the reference specimen.