Coarse aggregate is an important part of concrete material, and the change of its size will affect the contribution of aggregate interlock behaviour in concrete beams to shear capacity. In order to study the influence of coarse aggregate size on the shear failure and the size effect of shear strength of BFRP reinforced concrete beams, a meso-scale numerical model of BFRP reinforced concrete beams without stirrup was established. The comprehensive effects of size and coarse aggregate size on the shear failure mode and shear strength of BFRP reinforced concrete beams were simulated and analyzed. The study found that: the increase of the maximum aggregate size improves the shear capacity of concrete beam, and weakens the size effect of shear strength. According to the result of analysis, a theoretical formula of size effect on the shear strength of BFRP reinforced concrete beams was established on the basis of Fracture-SEL, and the accuracy and rationality of the formula are verified by comparing with the simulation results and experimental data.