A size setting method for a power switch transistor and a system thereof are proposed. A load current extracting step is performed to extract a first load current and a second load current. A limited voltage drop calculating step is performed to calculate a limited voltage drop according to a speed proportional value, the first load current and the second load current. A standard supply current calculating step is performed to calculate a standard supply current according to the limited voltage drop. A simulated supply current calculating step is performed to calculate a simulated supply current according to the standard supply current, the limited voltage drop and a line voltage value. A size setting step is performed to compare the first load current with the simulated supply current to calculate a size parameter, and set a size of the power switch transistor according to the size parameter.