If you look at a photo of the Big Boy you will see the graphite gray painted firebox area (originally silver paint was used) that extends from the cab to approximately the middle of the rear set of four driver axles. This firebox is incredibly huge (though amazingly it's not the largest). The forward part of the firebox actually extends into the rear of the boiler. This diagram might help.
I don't know if this image will be displayed full size or not. The preview shows it is set to fit the screen. I've provided a link to the full size image.