The smaller he bore the less peddle pressure is needed to increase the pressures in the wheel cylinders
Also the smaller the master cylinder bore the less volume per stroke
I would probally buy and install the smallest bore M.C. and verify that it was built for drum brake and disc brakes, verify that the reservior for front brakes has the 10 lb residual valve in place under the brass retaining plug and the rear reservior does not have a residual valve
Drum brakes usually need a 10 lb residual pressure to prevent the brake return springs from collapsing the brake cylinder in to far, the disc do not have that feature so no residual pressure needed as long as the M.C. is above the wheel cylinders, no gravity drain back.