Experimental　researches　on　the　behaviors　of　large-diameter　concrete-filled　steel　tube　(CFT)　columns　are　quite　limited　because　of　expenses　and　capacity　of　testing　equipment.　Finite　element　(FE)　technique　is　an　alternative　and　efficient　method　if　suitable　models　of　materials　are　available.　Many　FE　models　have　been　proposed　to　simulate　the　behaviors　of　CFT　columns.　Results　predicted　by　the　existing　models　are　generally　found　to　agree　well　with　the　experimental　results　of　CFT　columns,　the　diameters　of　which,　however,　are　generally　＜300　mm.　It　is　uncertain　whether　these　models　are　still　suitable　for　the　columns　with　diameters　larger　than　300　mm.　In　this　paper,　a　unified　FE　model　applicable　to　both　small-　and　large-diameter　CFT　stub　columns　is　proposed.　The　proposed　FE　model　is　then　used　to　study　the　influence　of　column　parameters　and　size　on　the　behaviors　of　steel　tube　and　confined　concrete　in　CFT　columns,　based　on　which　an　ultimate　strength　considering　size　effect　is　proposed.　It　was　found　that　the　existing　FE　model　predicts　conservative　results　for　large-diameter　columns,　whereas　the　proposed　model　performs　well　for　both　small-　and　large-diameter　columns.　Parametric　studies　suggest　size　effect　on　the　behavior　of　confined　concrete　in　a　CFT　column　is　more　significant　than　that　of　steel　tube,　which,　however,　is　mitigated　due　to　the　confinement　effect　resulting　from　steel　tube　when　compared　with　that　of　plain　concrete.　Finally,　the　proposed　ultimate　strength　model　is　found　to　perform　well　and　more　accurate　than　the　existing　model.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.