Several　size-effect　laws　(SELs)　have　been　proposed　and　verified　for　concrete-like　quasi-brittle　materials.　However,　there　is　no　SEL　that　can　quantitatively　describe　the　influence　of　related　factors　on　size　effect　in　carbon　fiber-reinforced　polymer　(CFRP)-strengthened　lightweight　aggregate　concrete　(LWC)　components　without　stirrups,　for　example,　the　shear　failure　of　beams.　The　main　purpose　of　the　present　study　is　to　develop　a　novel　SEL　for　the　evaluation　of　shear　strength　of　CFRP-strengthened　LWC　and　normal-weight　concrete　(NWC)　beams.　A　3D　mesoscale　simulation　method　is　employed　to　explore　the　structural　behavior　of　both　LWC　and　NWC　beams　wrapped　with　CFRP　sheets,　in　which　both　the　concrete　heterogeneities　and　the　steel/concrete　interactions　are　considered.　Based　on　the　modeling　approach,　the　influences　of　shear　span　ratio　and　CFRP　ratio　on　the　shear　failure　behavior　and　the　size　effect　on　shear　strength　are　studied.　Also,　the　shear　contribution　by　CFRP　on　the　LWC　beams　having　different　shear　span　ratios　under　different　structural　sizes　(i.e.,　beam-depth)　is　examined.　Finally,　a　novel　size　effect　law　that　can　quantitatively　describe　the　influence　of　shear　span　ratio　and　CFRP　ratio　on　shear　strength　of　CFRP-strengthened　LWC　beam　is　developed,　and　it　is　verified　with　the　available　test　data　and　simulation　results.