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

AJ/FormS is correct, and Physics works the same here in the U.S.

66fs poses a totally different problem:
If a clueless customer buys a new car and doesn't take it a brake shop until hearing metal scaping metal when braking, how big a reservoir is required so the inital fill of front brake fluid doesn't run out?

Of course, later cars added brake fluid level sensors, so they shouldn't reach such limits.

Actually, the flip-side is probably more important:
If during a brake job, the shop presses the caliper pistons back in, how large should an almost empty reservoir be so the brake fluid doesn't over-flow and ruin the firewall paint?