Optimization of spray combustion in diesel engines is one of effective ways for meeting future regulations for both pollutant emissions and fuel efficiency. Development of a new combustion system for diesel engines is usually expensive and time consuming, especially for large marine diesel engines. Since diesel engine researches are often repeated in different sizes that share similar characteristics, the ability to accurately predict engine performance from existing models would save both time and costs in engine development. Based on the similarity theory and conservation equations, some of the important dimensionless numbers in diesel spray combustion are deduced and discussed in this paper. Existence of similarity in diesel engines with different sizes are theoretically proved in diffusion (or mixing-controlled) combustion and premixed combustion as well as in spray mixture formation processes. With the prerequisite of geometric similarity, scaling rules for some parameters including engine speed, injection pressure and injection duration are established to realize the similarity between large-bore diesel engines and small-bore diesel engines. To verify the similarity theories, the computational fluid dynamics (CFD) simulation are conducted, and three scaling rules, which keep the engine speed, injection pressure and lift-off length constant, respectively, are compared under the conditions of light load (0.3 MPa IMEP), high load (1.55 MPa IMEP) and adiabatic homogeneous charge compression ignition (HCCI) operation. The theoretical analysis and simulation results demonstrate that the scaling rule that keeps the engine speed constant is more preferable under the operating conditions where premixed combustion dominates the heat release processes, while the similarity rule keeping the injection pressure constant is superior than the others when the diffusion (or mixing-controlled) combustion dominates the heat release processes. Nevertheless, the above three rules exhibit different degrees of similarity in terms of the NOx and soot emissions, and the discrepancy is discussed in depth.