Reinforced　concrete　(RC)　shear　wall　often　exhibits　a　brittle　failure　pattern,　which　may　present　an　obvious　size　effect.　The　objective　of　this　study　is　to　explore　the　size　effect　on　RC　shear　walls　with　shear　failure.　The　failure　of　RC　shear　walls　having　different　structural　sizes　and　aspect　ratio　of　1.0　was　simulated　by　a　two-dimensional　mesoscopic　simulated　method.　In　2D　simulation,　the　concrete　heterogeneities　and　concrete/steel　interactions　were　explicitly　considered.　The　mesoscopic　simulation　method　was　first　verified　with　the　available　experiment　observations,　and　was　utilized　to　study　the　influences　of　axial　load　ratio　and　horizontal　reinforcement　ratio　on　the　failure　of　RC　shear　walls　having　different　structural　sizes.　The　results　indicate　that:　1)　the　simulated　RC　shear　walls　present　brittle　failure　patterns,　exhibiting　obvious　size　effect;　2)　the　increase　of　horizontal　reinforcement　ratio　improves　the　shear　bearing　capacity　of　shear　walls,　while　it　would　weaken　or　even　suppress　the　size　effect　on　nominal　shear　strength;　and　3)　the　increasing　axial　load　ratios　improve　the　shear　bearing　capacity　and　lead　to　a　stronger　size　effect.　Moreover,　a　novel　size　effect　law　was　proposed　considering　the　quantitative　effects　from　horizontal　reinforcement　ratios　and　axial　load　ratios　for　size　effect.　©　2023　Institution　of　Structural　Engineers