Kenny,
I'd go ahead with the osage backing. Use no more than 1/8" thick, and make sure the grain runs as straight at possible down the entire length of it. It will work fine. As far as measurements, use the general sizes for an osage self bow for starters. Check the other threads about osage self bows to make sure I get this right, but I think 65 ntn, 1.25 wide tapering to 1/2" at the tips, 3/4" thick limbs to the fades, and 1" thick handle. Again, check these numbers with other threads to make sure I've got them right. It's been awhile!
Joe