Ghost,
I am 6'5" and use a full size FredT backplate. Fred's standard sizes are "full size" and "short". As he mentions above, he also does custom work. The full size plate works for me, although I am somewhat short-torsoed.
Trying one on for size at your LDS makes sense. If you do this, if at all possible, bring along your regular exposure suit and have it on when you try on the plate/harness. If not, wear something that approximates the thickness of your suit (coat, etc.).
When I have my suit and harness on, the harness belt is higher on my body than a normal dryland belt and also slightly higher than my weight belt (on my hips). This works for me, since it helps me avoid confusing my harness belt for my weight belt underwater. As a result, it may be worthwhile to bring your weight belt along to try on with the BP. FWIW, I wear my weight belt under my harness.
As for BP length, the length of the plate will determine the position of the harness belt in relation to your body. The longer the plate, the lower the belt will be, since it is threaded through slots in the bottom corners of the plate (see attached file for picture of FredT plate).
Hope this is somewhat helpful.
-LD