If you have reference images of what you want, you can try analyzing them to find dimensions. Basically, open the image in something like Photoshop. Then find details near the window that you can start to use assumptions on their width or height. In my example below, I was initially looking to figure out the height of the header above the door using the base tile height. Later, I used perspective lines that informed me of the height and dimensions of other details.
As I was doing this, I was changing the sizes of my 3d models to see if the scale looked similar to the picture by placing my camera in a similar position. In case you're wondering, the base tile turned out to be 8"h, making the door header 110" A.F.F., and the wall tiles a mix of 16x16" and 8x16" tiles, making the wall 120" tall (not including the 4" cap), and the green beams 14"h.
Luckily, most major structural details in architecture measure to nice round numbers!
Good luck!