This study investigates the impact of compressor size on the seasonal coefficient of performance (SCOP) in heat pump and air conditioning systems for two different regions, Rochester, Minnesota, and El Paso, Texas. A single-family home was simulated in EnergyPlus and heating and cooling loads for the entire year were obtained on hourly basis. The compressor type was a rolling piston compressor. The compressor design was a load-based design which means the compressor was designed based on the peak loads. The Rochester MN compressor was designed based on the maximum heating load and the El Paso TX compressor was designed based on the maximum cooling load. Additionally, three refrigerants—R410A, R290, and R1234yf—were analyzed, each with its own unique design for each location, resulting in six total load-based designs. The compressor performance was simulated using PDSim, an open-source compressor simulation package which works based on mechanistic chamber modeling approach. The size of each compressor varied from 50 to 125% of the base design and SCOP for each size was obtained based on hourly data for the entire year. The results showed that the optimal size for each compressor design varied depending on the refrigerant used and the target location. For example, in Rochester, MN, the optimal size for the R410A compressor was 65% of the load-based design. For R1234yf, it was 50%, and for R290, the optimal size was 70% of the load-based design.