Hello, I think that another consideration for you is that the camera you are using will determine a lot about the image. If you are wanting a 12x20 image, then the best way to get that is to use a 12x20 camera. If you are using an 8x10 camera, you will tend to want to fill the 8x10 frame. But of course you can set up any camera with masking so that you only see on the ground glass an image that is porportional to 12x20.
I mention this because I used 4x5 for several years before I was able to move up to 8x10. I then added 11x14 and 12x20. I was struck with each new film format how different it is from the others. When I am carrying a particular camera I am trying to fill the frame of that particular format.