This study aimed to downsize bi-propellant rocket engines by utilizing detonation combustion characterized by significantly fast combustion completion. In rotating detonation engines (RDEs), which are a type of rocket engines utilizing the detonation combustion, the combustors typically had annular configurations and consisted of two coaxial cylinders to stabilize detonation waves within the combustor. However, through investigation of the effects of combustor sizes on the engine performance, it has been revealed that a simple cylindrical configuration, in which the combustor consists only of one cylinder, can exert equivalent engine performances to conventional annular combustors by setting the propellant flow rate appropriately for the combustor cross-sectional area.