Rule of thumb?

[As2]

How much insulation to use in a steel building regardless of size?

[MikeT]

If the building is heated and cooled get the highest R-value or thickness. Common is 4 inch for both walls and roof. If you use 6 inch - thin out the insulation over girts and purlins to lessen the "pillow effect"---you don't want panel bulging. Specific commercial codes can be met by going to a banded system that puts it between the purlins and not as much between purlin and panel.