Numerous　size　effect　laws　of　concrete　materials　have　been　proposed　by　scholars,　while　the　size　effect　laws　for　concrete　filled　steel　tubular　members　are　still　under　study.　A　three-dimensional　meso-scale　numerical　model　that　can　consider　the　heterogeneities　of　concrete　and　concrete-steel　tube　interactions　was　established　to　study　the　size　effect　in　concrete　filled　steel　tubular　columns.　The　failure　behavior　of　axial-loaded　circular/square　concrete　filled　steel　tubular　columns　having　different　structural　sizes　under　different　lateral　constraints　(represented　by　confinement　coefficients)　were　investigated　based　on　the　meso-scale　method.　The　influence　of　lateral　confinement　on　nominal　axial　compressive　strength　for　concrete　columns　and　corresponding　size　effect　were　also　explored.　As　the　simulation　results　indicating,　with　increasing　the　specimen　size,　the　nominal　axial　compressive　strength　decreases,　and　size　effect　is　weakened　as　the　confinement　coefficient　increases.　Moreover,　combining　with　the　influencing　mechanism　of　steel　tube　on　compressive　strength　for　axial-loaded　concrete　column,　a　theoretical　formula　that　can　describe　the　influence　of　lateral　constraint　on　size　effect　was　established　according　to　the　available　size　effect　law　for　concrete　materials.　Comparing　with　the　simulation　results　and　reliable　test　data,　the　accuracy　and　rationality　of　the　theoretical　formula　is　well　verified.