NOVELTY - Optimizing size of fuel processor comprises: (i) sizing first carbon monoxide reduction reactor to reduce carbon monoxide to first set level when fuel processor operates in a most-used power range; and (ii) sizing second carbon monoxide reduction reactor to reduce he carbon monoxide concentration output to less than second set level when the fuel processor operates at the maximum power level.    USE - Optimizing the size of a fuel processor for use in a fuel cell system e.g. for electric vehicle.    ADVANTAGE - The method yields a fuel processor design that has a peak efficiency in a most-used power range, i.e. the fuel processor maximizes the steam/carbon ratio which thermally balances the most-used power range to optimize performance in that power range and to obtain low CO levels from the water gas shift reactor (22) for increased efficiency.    DETAILED DESCRIPTION - Optimizing the size of a fuel processor having a primary reactor (12) and two carbon monoxide (CO) reduction reactors comprises:    (a) determining a most-used power range of the fuel cell processor;    (b) maximizing water availability in the fuel cell system at the most-used power range;    (c) sizing the first CO reduction reactor to reduce CO to a first predetermined concentration when the fuel processor operates in the most-used power range; and    (d) sizing the second CO reduction reactor to reduce the CO concentration output by the first CO reduction reactor to less than a second predetermined level when the fuel processor operates at the maximum power level.    An INDEPENDENT CLAIM is also included for a fuel cell system comprising a fuel cell (10) operable to provide electrical power in several power ranges, and a fuel processor for providing hydrogen to the fuel cell.    DESCRIPTION OF DRAWING(S) - The figure is a simplified schematic diagram depicting a fuel cell system.    Fuel cell (10)    Primary reactor (12)    Fuel stream (14)    Steam stream (16)    Air stream (18)    Water gas shift reactor (22)