In　Split　Hopkinson　Pressure　Bar　(SHPB)　tests,　the　size　effect　is　a　significant　factor　influencing　the　dynamic　mechanical　properties　of　concrete-like　materials.　To　investigate　the　impact　of　size　effect　on　concrete　strength,　this　study　conducted　dynamic　compression　tests　on　concrete　specimens　of　different　sizes　using　a　Phi　74　mm　SHPB　test　device.　The　results　indicated　that　concrete　material　exhibited　a　significant　strain　rate　effect　and　size　effect.　Based　on　test　results,　the　Holmquist-Johnson-Cook　(HJC)　model　considering　the　strain　rate　effect　was　used　for　numerical　analysis,　with　improvement　made　to　the　strain　rate　effect　term.　Different-sized　numerical　simulation　models　were　established　using　Abaqus　software,　and　parameterized　analysis　of　specimen　sizes　was　conducted　based　on　theory　and　simulation　results　to　determine　the　optimal　specimen　size.　Based　on　the　analysis　results　and　considering　the　synergistic　effect　of　strain　rate　effect　and　size　effect,　a　size-modified　coefficient　was　introduced　into　the　constitutive　model　to　establish　a　calculation　model　for　the　size　effect　of　dynamic　compression　in　concrete.　The　model　considering　the　size　effect　could　effectively　predict　test　results.