....from Model Railroader magazine March 1954. These plans are still good for HO scale. They are inadequate for live steam or very large scales that require more details on the engine assemblies (the two sets of driving axles). Supposedly the Union Pacific Museum sells a CD set with several plans on it. I don't have a set for the Big Boy, but I do have a large book that has all the engineering sheets for the 4-8-4, of which 844 is still running. There is enough detail in that book to accurately make every part. Also have the factory manual with drawing sets for the Northern Pacific Baldwin VO-1000 diesel switch engine that my grandfather was injured in a wreck on, resulting in a medical retirement. I would love to make a large model of it some day.
Anyway, viewing these plans in conjunction with the one in the previous post will show the firebox proper extending to approximately the middle of the rear engine truck, with a firebox extension into the boiler going a little further for complete combustion in the boiler.
The Northern Pacific Yellowstone class 2-8-8-4 actually had a larger firebox, but it was designed for a grade of coal that required more surface area.