Lots　of　experiments　observe　the　size-dependent　phenomena　of　concrete　mechanical　behaviors.　In　this　paper,　the　micropolar　theory　is　adopted　to　describe　the　size　effects　by　introducing　two　size-related　parameters　into　their　constitutive　equations,　named　coupling　number　N　and　characteristic　length　l.　A　micropolar　damage　model　is　built　for　size-dependent　concrete　fracture　problems　utilizing　the　bond-based　peridynamic　(PD)　idea.　That　is,　the　material　damage　and　fracture　behaviors　are　determined　by　the　local　damage　factors　of　the　PD　bonds.　Then,　the　tensile　strength　of　a　concrete　specimen　is　numerically　estimated　by　the　PD　differential　operator　(PDDO)　method.　The　influences　of　the　size　parameters　N　and　l　on　the　tensile　strength　are　studied.　By　comparing　with　the　transformed　Bazant　size-effect　law　and　the　fitting　error　analysis,　the　reasonable　values　of　N　and　l　are　determined　for　a　certain　reinforced　concrete　composite.　Finally,　by　using　the　determined　micropolar　damage　model,　the　crack　propagation　paths　in　concrete　members　are　numerically　simulated.