The discrete element method (DEM) was used to analyze the effect of specimen size on the mechanical behavior of basalt rockfill materials in order to find the representative elementary volume (REV) of the strength and deformation parameters of the rockfill materials with laboratory scaled gradation. Laboratory single-particle crushing tests were conducted to analyze the crushing characteristics of basalt particles and determine the particle crushing strength parameters required for DEM simulations. A series of triaxial compression DEM tests with different specimen sizes, particle arrangements and confining pressures were conducted. The variations of strength and deformation parameters with specimen size were analyzed for rockfill materials, and the REV of each parameter was assessed. Results show that particle arrangement and specimen size both impact the mechanical behavior of basalt rockfill materials. The variation coefficient of each mechanical parameter decreases as specimen size increases. The REV size of different mechanical parameters is different. The appropriate specimen size can be selected according to the research object in order to improve the calculation efficiency of DEM. The size of the numerical sample should not be less than 300 mm in diameter and 600 mm in height in order to make all parameters stable under the simulated conditions.