Numerous　studies　have　indicated　the　existence　of　a　size　effect　on　the　axial　compressive　behaviour　of　stirrup-confined　concrete　columns.　However,　most　of　these　studies　have　focused　on　nominal　compressive　strength.　Investigations　of　the　size　effect　on　the　axial　strain　(at　peak　load)　and　of　the　descending　branch　are　limited.　In　this　study,　the　size　effect　behaviour　of　square　stirrup-confined　concrete　columns　under　axial　compression　was　explored,　using　a　three-dimensional　mesoscale　simulation　method.　Based　on　the　numerical　and　available　experimental　results,　the　influence　of　specimen　size　on　peak　axial　stress　(i.e.　compressive　strength),　corresponding　strain　and　softening　rate　was　explored.　Moreover,　quantitative　relationships　between　specimen　size　and　peak　axial　stress,　corresponding　strain　and　softening　rate　for　circular　and　square　stirrup-confined　concrete　columns　were　derived.　Finally,　considering　the　size　effect　on　the　peak　axial　stress,　corresponding　strain　and　softening　rate,　a　novel　stress-strain　model　describing　the　axial　compression　behaviour　of　stirrup-confined　concrete　was　developed.　The　proposed　model　was　verified　by　comparison　with　the　available　experimental　results　and　existing　models.　©　2021　ICE　Publishing:　All　rights　reserved.