Recently, demand for small-bore compact vehicle engines has been increasing from the standpoint of further reducing CO2 emissions. The generalization and formulation of combustion processes, including those related to emissions formation, based on a certain similarity of physical phenomena regardless of engine size, would be extremely beneficial for the unification of development processes for various sizes of engines. The objective of this study is to clarify what constraints are necessary for engine/nozzle specifications and injection conditions to achieve the same combustion characteristics (such as heat release rate and emissions) in diesel engines with different bore sizes. Firstly, geometrical similarity theory was applied to two single-cylinder engines with different bore sizes but geometrically similar specifications and configurations, including the compression ratio, bore/stroke ratio, piston cavity shape, and nozzles, to confirm whether the engines have similar combustion characteristics. Additionally, the spray was visualized using an optically accessible vessel for more detailed analysis of the similarity of spray characteristics. In the next step, a new approach called spray characteristics similarity theory was developed to improve the NOx-smoke tradeoff in smaller size engines. Theoretical considerations indicated the existence of solutions for nozzle orifice size, orifice number, and injection pressure as functions of the similarity ratio. These solutions satisfy the similarity of spray characteristics regarding air-entrainment and penetration with different bore sizes. Experimental investigations clarified that the nozzle specifications and injection conditions derived from the proposed similarity theory can recover the NOx-smoke trade-off in a smaller engine to almost the same level as a larger engine. Finally, the spray characteristics theory was modified considering the effect of the flame lift-off length. This modification can completely eliminate the small NOx-smoke trade-off difference between the L type and S type engines. Consequently, the spray penetration and excess air ratio at the flame lift-off length were found to be key factors for maintaining the same combustion characteristics in different sized engines.