Both are used.
The injector need to flow the required amount of fuel to support a maximum HP while maintaining an "on" time of not more the about 80%. This keeps them from going static or on all the time.
The engine displacement also comes into play when calculating the proper BPW-base pulse width that the computer uses as the baseline injector pulse. Then it adds and subtracts from that.
Now it's fairly easy to make a larger injector from a smaller one by simply increasing the fuel pressure at the regulator. (see the link earlier in this thread)That way the injector sprays more fuel each time it open for the same amount of time. It also helps with the atomiztion of fuel.
GM did just this in 1994 when they changed the 454 TBI fuel pressure from 13psi to 25psi.
In doing so they went from a 80#hr injector to a 50#hr injector. But by increasing the pressure it now flows the same as the 80.
But like I stated earlier, chose a system from a donor that most closely matches yours.
The big 4 or small 6 can go either way but then the netres chip comes into play if you want an accurate limphome.
