Will still not leave this unanswered 
My frame is 3500mm ~ 11.48’ wide. My motors are 302cm ~ 9.9’ apart. I wanted to stay close to the stock distance and kept some space to play with.
Only my personal believe: I think quotes like this and that there is no ideal ratio between hight and width are the result of that we just have to many variables to calculate that precisely. In my ideal dream world, it must exist. It is near a ratio of 2:1 is my superstition. If your faith can be a substitute for pure facts and science, you could try to have the motor width at 11’ and the bottom beam, were your sheet rests on, at 5.5’. The effusions of a drunken man about this can be found here: Is there an optimum ratio between width and hight?