I second iceblossom's suggestion to take graph paper and draw what looks appealing to you.
In real life - many attic windows seem very small in comparison to the walls.
you could also cut strips of paper and lay them around your blocks to get an idea of proportion. Remember to add seam allowance
A lot of people do have design programs.
A lot of us still use graph paper.
Some pelple just lay fabrics out until they get an AHA moment.
A design wall - or bed - or floor - is also helpful.
When using a design wall - take pictures of each layout before changing it. Somehow a camera seems go see things our eyes and brains miss.
To answer your original question - I am not aware of a hard rule for proportions.