Reinforced　concrete　(RC)　columns　are　prone　to　shear　failure　under　seismic　loads,　particularly　in　the　case　of　short　columns.　However,　this　brittle　failure　may　have　more　adverse　effects　on　large-sized　columns.　To　address　this　issue,　a　common　solution　is　to　retrofit　RC　columns　using　fiber-reinforced　polymer　(FRP)　laminates　to　improve　seismic　performance.　The　primary　objective　of　this　study　is　to　assess　the　shear　performance　and　size　effect　of　rectangular　RC　columns　confined　using　carbon　fiber-reinforced　polymer　(CFRP).　To　account　for　the　heterogeneity　of　concrete,　a　mesoscale　numerical　approach　is　developed　using　a　random　aggregate　model.　The　investigation　focuses　on　impacts　of　the　axial　compression　ratio　and　CFRP　volumetric　ratio.　Results　show　that　the　cross-sectional　size　has　little　effect　on　the　final　failure　mode　of　the　columns.　However,　the　width　of　the　main　diagonal　crack　is　reduced　as　the　cross-sectional　height　increases.　It　can　be　observed　that　the　size　effect　on　the　CFRP　rupture　hoop　strain　distribution　is　primarily　manifested　in　the　strain　value　rather　than　the　shape　of　the　distribution.　The　size　effect　is　evident　in　the　total　shear　strength　of　columns　and　in　the　CFRP　shear　contribution,　with　the　effect　becoming　more　pronounced　with　increasing　axial　compression　ratio.　Taking　into　account　the　influences　of　size　on　the　CFRP　shear　contribution　and　shear　strength　of　RC　columns　as　well　as　the　CFRP　confinement　effect　on　concrete,　the　calculation　model　of　shear　capacity　is　established　based　on　a　new　proposed　FRP　effective　strain　model　available　for　rectangular　columns.　The　proposed　model　can　provide　a　more　accurate　prediction　on　shear　capacity　of　rectangular　CFRP-confined　RC　columns　compared　with　existing　design　codes.　©　2023　American　Society　of　Civil　Engineers.