Currently working on a model based around the dimensions found in the Santa Fe System Stds Vol3.
These boiler (not power houses) houses were mainly cast concrete structures with outer wall doors at the front and rear of each boiler in the 2 boiler house plan. The building was 29'6" long by 24' wide. The side walls had two windows on one side and a single window and door on the other side. The concrete floor had a 2 foot pit with sloping sides for each boiler. The outlines of the boilers show them about 19' long, 4'6" dia boiler tube, 5' wide firebox.
A power house included the boiler area, plus the power generation machinery area. The buildings were also cast concrete, but with a greater window area. I'm working from a pair of terrible photos to build a model of the 4 boiler power house that was behind the Slaton, Texas roundhouse. Hope this provides some help.

Robert