Counter balance custom use

I'm a architect/wood worker and I'm building a frame for a over/under sliding whiteboard. Do you think I could use your spiral balances to counter balance the white boards? They will be 5'W x 4'H. Next question is how to do you measure the size needed? Does the balance body need to match the height of the "window" or can it a portion of it and then live with whatever extension is available (ie if no 48" long ones exists?) --Brian