The　available　researches　on　concrete　columns　confined　with　fiber-reinforced　polymer　(FRP)　laminates　have　not　comprehensively　addressed　the　size　effect.　In　this　study,　a　three-dimensional　mesoscale　simulation　approach　that　can　consider　concrete　heterogeneities　was　established　to　explore　the　size　effect　of　glass　FRP　(GFRP)-confined　concrete　columns　under　axial　compression.　The　influence　of　constraint　ratio　and　cross-sectional　type　on　the　failure　of　columns　was　investigated.　In　addition,　the　effect　of　lateral　constraint　on　the　nominal　compressive　strength　and　the　corresponding　size　effect　was　quantitatively　studied.　The　simulation　results　indicate　that　structural　size　has　a　significant　influence　on　the　nominal　compressive　strength　of　GFRP-confined　concrete　columns.　For　the　present　circular　columns,　size　effect　on　nominal　compressive　strength　is　weakened　or　even　suppressed　as　the　FRP　constraint　ratio　increases.　For　square　columns,　the　presence　of　GFRP　cannot　suppress　the　size　effect　on　compressive　strength,　since　it　cannot　provide　sufficient　constraints.　Furthermore,　a　size　effect　formula　that　can　quantitatively　describe　the　influence　of　lateral　constraint　on　the　size　effect　of　GFRP-wrapped　circular　concrete　columns　was　developed.　Good　agreement　between　the　theoretical　results　and　the　simulation　results　as　well　as　the　available　test　results　confirms　the　rationality　of　the　developed　size　effect　law　(SEL).