You need to get a booster from a early model AE86.
Then you need to measure the piston sizes of your big brakes, and the original brakes.
Then measure the AE86 master cyl diameter, or read on the side that it's 13/16".
Then select a new master cyl size that you want, but keeping the brake to master area ratio about the same. It is not just the diameter ratios, but the area ratios. So pie dee squared over four.
For a fixed caliper, you only consider pistons on one side for the calculation.