Toot, the roofing like that was called tin roofing around here. It was 8' long and 36" or 48" wide, depending on what you got from the lumberyard. One thing that will help is knowing what "G" scale he has. G scale covers 1/20, 1/22, 1/24, 1/28, and as well as a few oddball scales thrown in there as well.
That is one of the "problems" associated with G scale at it's inception. The NMRA said it would set standards and the manufacturers objected, saying they would set all standards needed. This conglomeration is the result of those manufacturers, "setting" the standards.