[Dave at Pelican Parts]

The size of the MC and size of the calipers are only somewhat-related. In general, to keep a similar feel to what the car already has, the ratio of the total piston area of all of the pistons in all of the brake calipers to that of the master cylinder should be the same as in the stock system. Yeah, that's a bunch of math... But the calipers don't care what size MC is providing them with pressure, all they care about is the pressure in the brake lines.

A 20mm MC is available; it may have come in some 911SCs, but I am not certain. There is a 21mm one and a 23mm one from Mercedes. I do not know the models.

The larger the diameter of the MC, the harder the brakes will be to push. Conversely, the larger the total diameter of the brake caliper pistons, the easier the pedal will be to push. (And the more fluid you'll need to move.)

I recommend going five-bolt, getting either M or A caliper 911 brakes, and using the 19mm MC. That ought to be enough for any reasonable street use. If you're tracking the car (Big Track, not autoX) then you have different needs and should pay a lot of attention to brake cooling.

--DD