Reverse osmosis desalination plants running with renewable energy-based microgrids are gaining importance. A new approach for the design of microgrid with a reverse osmosis desalination plant system called coordinated sizing is proposed in this manuscript. The proposed algorithm is a leader-follower structured optimization process. The desalination plant is segregated as units and each unit can be operated independently as required. A direct load control-demand side management is incorporated into the sizing process to determine the optimal flexible operation of the desalination plant and microgrid's power dispatch simultaneously. A conceptual microgrid powering desalination plant is considered to evaluate performance of the proposed coordinated sizing method. Different evolutionary algorithms are applied for solving the sizing problem and the results are analyzed. The proposed coordinated sizing method is compared with the conventional sizing method. With and without the flexible operation of desalination plant cases are considered to compare economy of the plant. The results demonstrate that the proposed coordinated sizing is cost-effective and provide better operational flexibility. © 2020 Elsevier B.V.