While discussing what affects the amount of pressure exerted by the brakes: Technician A says that the shorter the line, the more pressure there will be. Technician B says that braking force will increase if the size of the pistons in a master cylinder are increased. Who is correct?

Only Technician B is right.

Explanation:

The cylindrical braking system for a car works through the mode of pressure transmission, that is, the pressure applied to the brake pedals, is transmitted to the brake pad through the cylindrical piston.

Pressure applied on the pedal, P (pedal) = P (pad)

And the Pressure is the applied force/area for either pad or pedal. That is, P (pad) = Force (pad) / A (pad) & P (pedal) = F (pedal) / A (pedal)

If the area of piston increases, A (pad) increases and the P (pad) drops, Meaning, the pressure transmitted to the pad reduces. And for most cars, there's a pressure limit for the braking system to work.

If the A (pad) increases, P (pad) decreases and the braking force applied has to increase, to counter balance the dropping pressure and raise it.

This whole setup does not depend on the length of the braking lines; it only depends on the applied force and cross sectional Area (size) of the piston.