Abstract: In order to investigate the effect of structural dimension on the bending-shear-torsion composite mechanical properties of BFRP-reinforced concrete (BFRP-RC) beams, a numerical model of BFRP-RC beams under combined loading was established by using a 3D mesoscale simulation method. The numerical method was used to analyze the effects of structural dimension, shear span ratios and torsion-bending ones on the mechanical properties and the corresponding size effect law of BFRP-RC beams under composite loading. In addition, the size effect law proposed by Lei et al. for BFRP-RC beams under bending-shear-torsional composite loadings was modified. Research shows that: there is a mutual weakening effect between shear and torsion of BFRP-RC beams; BFRP-RC beams have obvious size effects under bending-shear-torsion composite loading, moreover, the torsional and shear size effects are firstly enhanced and then weakened as the torsional ratio increases; shear span ratio has a significant effect on beams’ shear and torsional strength, but it has basically no effect on the corresponding size effects; the modified size effect law can accurately predict the influence of member size, torsion-bending ratio, shear span ratio and stirrup ratio on the torsional and shear capacity of beams under composite loadings.