You can get images in SU to scale with absolute accuracy...but you need to import them as images, not textures.
You then draw a line on the image of known length, such as a line across the top of a window frame, obviously, the longer the known length, the more accurate it's going to be...then group the line and image together.
Entering the group, you then use the tape measure tool to measure the line. Ignore whatever value appears in the VCB, but type in the actual length and hit Enter. You will the be asked if you want to resize the contents of the group....obviously yes.
Having now sized the image with absolute precision, you can place it in front of the appropriate section of the building, explode it, sample it and then paint it onto the wall.
