What if I don't change my Master Cylinder on a disc conversion?

The size of the reservoir is not an issue. Once the system is bled, and the fluid topped up, the reservoir level will drop very little when the pedal is depressed (as soon as the pedal begins to move the piston the seal passes the reservoir port isolating the fluid in the reservoir from the fluid in the lines, if it didn't you would not be able to build pressure in the lines). Also, pre-73 disk equipped cars generally had same sized reservoirs from the factory, the larger front brake reservoir is there for convenience and safety. As the pads wear the level will drop. Having a larger reservoir means you don't have to top off the fluid as often.

You should be more concerned with residual pressure valves in a master cylinder that was originally on a drum/drum car. The valve (if present) can be easily defeated with appropriately sized drill bit.

A commonly available 15/16" bore master cylinder should work fine. Most of the new or rebuilt master cylinders out there are based on the large/small reservoirs. So unless you want an equal reservoir master cylinder for visual appearance, it's probably easiest to swap to a rebuilt disk/drum part.