"The sixth step should be placed slightly across the body. What exactly does that mean to you. It has to be of a predetermined length depending on step five. I was always told that to go for any kind of length on step six will kill the CBMP and will not allow me to drive on the next heel lead. Thus losing light and shade."
Indeed, the sixth step is placed slighlty across the body. However, on a fully skilled dancer, the body has never stopped moving and travels quite a bit during the sixth step, so placing the foot "slightly across the body" ends up meaning quite a bit of floor is covered - the little bit that is the distance across the body, plus the large amount that the body has moved during this step.
If you merely tried to place the leg into a large step across the body, then yes that would ruin your CBMP. But if most of the distance comes from the continued movement of the body, and only a minority of it from the placement of the leg across the body, then you can have a moderately large step in perfect CBMP. Beginners aren't ready to move the body like that, so they are taught to make it a small step.
Step five does determine the lenght of step six, but it determines it most by the amount of rise achieved - the higher you rise, the larger step six will be, because the greater the distance the body will cover during the course of lowering. If you tried to place step 6 short of where your body flight was taking you, you would fall past the step rather than through it, and you would stumble into the next figure rather than being able to do a nice drift+drive action. Remember that your weight must pass through the heel of that foot on step 6 - if it goes directly to the foot, your have over-run the foot placement and killed the elegance of your dancing. Chances are, you will crush your partner into the floor on the next step.
Good dancing requires the the rise and body travel be perfectly in proportion to each other - and these will entirely deteremine the placement of the free foot.
If you are thinking and placing your foot with intention, you are playing connect the dots and not yet dancing.