Are you building a flat, or just making an image for the back drop?
If you're building a flat, you're going to be beholden to commercially available window castings, so you would adjust your height and width to keep things proportional.
If you have a program that lets you draw to dimension, I would just guesstimate it to start, then monkey around with it until it looks right.
Massing and proportion are more important in building a presentational model than raw dimensions.
Lee