This has been answered already since the car factory system comes in variable shapes and sizes, and so do the speakers. You have to be very clear on which size of the Speaker you require to go with.
This selection is crucial because it impacts the quality of sound your amplifier will produce. Car speakers come in different sizes because not all car speakers get fitted in the car.
If you picked the wrong size, you would probably need to modify the speakers’ port to get them fit in, and for this reason, car speakers come in variable sizes.