Sorry, this may confuse matters, but bore size is not the only consideration. There is also the lever fulcrum and arm moments to consider when seeking hand pressure changes.
The cylinder swept volume does increase when increasing bore diameter, as it can move more volume for the same stroke length as the smaller bore.
But, the volume comes at a cost when looking at the pressures achieved. To create the same hydraulic pressure at the caliper, more pressure has to be applied to the master cylinder piston.
This is where the lever arm moments come into focus. These moments determine the mechanical advantage of the lever, about the fulcrum/pivot.
Length of hand lever vs, length of piston actuating arm.
A dual disk arrangement would need more fluid volume to actuate than a single disk if the caliper cylinder diameters were the same (piston diameter) in both. This favors a larger diameter master. However, the pressure requirements are a different matter. To double the pressure, would require a smaller diameter master cylinder piston, traveling twice as far, to get the volume needed for caliper cylinder movement. That's the physics of the hydraulic side of the system.
The hand pressure applied to the master cylinder piston, is determined by the lever arm moment ratio, lever length to piston lever length.
I don't think you can pick a satisfactory master cylinder without knowing the hand lever ratios. Unless, luck plays a factor.
Cheers,
