Energy management systems (EMS) and autonomous power control (APC) for space microgrids (MGs) on the Moon need properly designed operating points and references to ensure the mission's safety. The oxygen and water requirements of the lunar base are supplied by the In-Situ Resource Utilization (ISRU) from the lunar regolith. ISRU is one of the most power-demanding subsystems in the lunar base. This paper proposes an optimization methodology for sizing and optimal operation management of a photovoltaic (PV)-battery-based space microgrid (MG). By solving the optimization problem, the optimal size of the PV array and battery, as well as the PV power generation and battery charging/discharging profiles are determined. First, the ISRU power demand profile is presented considering the oxygen and water management systems of the lunar base. Then, the optimization algorithm is employed to minimize the PV and battery mass and the total unused PV power generation while maintaining the desired level of energy in the battery considering system constraints. It is observed that curtailing the excess PV power generation plays a crucial role in minimizing the battery size and mass, thereby reducing the cost of the space mission.