We must not get confused with pressure versus force. Effectiveness of the brakes depends on the force being applied to the brake shoes. You can put what size of slave cylinder you like and it won’t affect the pressure. The pressure is created by the piston in the master cylinder, acting on the volume of fluid in the pipes etc. What does change with different sized brake cylinders, is the FORCE acting on the brake cylinder piston.
Force=Pressure x Area, so for a constant pressure applied via the brake pedal, increasing the piston diameter of the brake cylinder will increase the force acting on the brake shoes.
