This is from Tim Baker from Paleoplanet
"River birch is about .56 specific gravity. It's one of the lighter birches, So for example if making a 65" bow with 6" of net stiff handle area, a 50lb bow at 28" should be almost 2" wide out to mid limb. If fairly highly crowned go narrower and longer. If making a D-bow, 74" long and 1 3/8" wide from midlimb to midlimb should be safe if tillered well. "Tim
Eric