Hi Mrs. Green!
Hexagram sizes should be proportionate to the room. When we do an event in a larger room I generally put an 8-1/2 X 11 hexagram above the door. For a large room in a home I will use a half sheet and for a smaller room a quarter sheet. The more you use them, the more you will get a feel for what is the ideal size.
Don't forget, too, that you can muscle test to see what is most beneficial, not only for hexagrams, but for the sizes and placements of all your activations and cures. If you have the Diamond Dowsing course, you could also dowse to see what would be the most powerful energetically.
I prefer to see hexagrams, because they remind me of what I am activating. Some customers prefer to put them behind other items, but I would definitely suggest muscle testing to make sure this is as effective as having them in sight.
Doing feng shui activations is very akin to creating a vision board of your home, and if you can't see what you are envisioning I would think that effect would be muted. That being said, though, if the choice is between not placing a hexagram or putting it behind something, I would definitely do the latter. Like everything else, it's a matter of "good, better, best."
Enjoy your course and the great energy you are creating for yourself and anybody else who enters your space!