spending the money to upgrade at all? I could design 10 different setups, but that is too much time. 12.5" sounds good to me since it includes the stock wheel guys, but I am looking at a 13" based on 4 piston calipers also.
And for you guys that criticize brake kits that are not tuned to the 300zx, the stock front to rear brake torque % will remain close to stock. This means that if the front calipers have 50% (just a random number) more piston area, the rears will be very near the same increase in piston size.