In the present study, a multi-criteria sizing function (MCSF) is proposed for designing the optimum size and operating strategy of the prime mover of a residential micro-combined cooling heating and power (CCHP) system. The CCHP prepares the electrical, thermal, cooling, and domestic hot water demands of the same building in five different climates in Iran. The MCSF integrates fuel energy saving ratio (FESR) and exergy efficiency as the thermodynamical parameters, net present value, internal rate of return and payback period for the economical criteria, and CO2, CO and NOx reduction for the environmental evaluations. Analytical hierarchy process is used to weigh each criterion with respect to others. The engine proposed by MCSF results in considerable fuel saving and pollution reduction and a payback period of about 6 years for the 5 climates. In addition, the best strategy according to the engine size is determined for every climate. (c) 2013 Elsevier Ltd. All rights reserved.