In this study, a fundamental generalized thermodynamic model of internal combustion engines was applied to evaluate engine compression ratio effects principally in relation to engine size. Performance and efficiency metrics were investigated systematically. Further, cylinder wall temperature was varied across a range of cold start to stabilized operating temperatures. A very broad range of engine bore sizes and bore-to-stroke ratios were evaluated, representing small to large diesel engines in service. In general, it was observed that engine efficiency increases moderately with increasing compression ratio and bore size. Additionally, surface area-to-volume ratio is a critical metric when evaluating various size engines. This leads to greater relative heat transfer in the smaller bore engines with higher compression ratios. The sensitivity to heat losses is also much greater in the smaller engines. Smaller engines with higher compression ratios are expected to be most affected by cold starting conditions. Exhaust enthalpy is highest for larger bore engines with lower compression ratios, an important consideration for engine boosting. Higher convective heat transfer coefficients are also expected in smaller bore engines with higher compression ratios due to the higher operating pressures.