Pressurized fuel will flow through the rail, which is delivered to the injectors. The pressure will pop the rail of the injectors if you don't bolt it down.

The regulator doesn't have to be adjustable. It will simply maintain a constant pressure of about 3 bar in the rail. It does however have a pressure input, which will rise the pressure when you add boost, or lower it when vacuum occurs. I run an unadjustable 2 liter Mazda one on mine, which I got of a manifold I have, and it works perfectly for my application. I only used it because it looked better(read newer) than the Toyota one

Other than that the fuel pressure has nothing to do with the size of the engine, the amount of fuel delivered is controlled mainly by the management system, and the pressure only plays a small role where sane NA or light turbo applications are concerned. However you still need good control over the pressure to keep it constant, otherwise it will be very difficult to set the management. It's only when you start going to insane boost levels that you really need an adjustable regulator, where you start relying on the fuel pressure to inject enough fuel.