for a pyramid design you typically go with something like 2 inches tapered to 1/2 inch down the entire length, it takes advantage of woods strength to width ratio, which makes it easier to tiller because the whole thing is one thickness throughout. with the width you have already cut to youd have to make your tips something like 3/16, which is really narrow. i suggest just looking around at some posts and maybe reading some of The Bowyers Bible to get ideas for exact thicknesses, then just go with what you think is best. if you go slowly im sure youll be ok, osage is known for being pretty resilient even with knots and holes through the limbs.as far as length ill simply say this, having a shorter bow really only provides convenience, as in ground blind hunting, or moving through brush. the extra inches(say if you make it 66"" will help to keep your areas youre worried about a bit safer and give the bow more forgiveness. i havent built a bow shorter than 66 inch so far because i still question both my patience and my tillering abilities, which sometimes go hand in hand. if all else fails and you still are unsure then just pm one of our gurus here, im sure theyd be more than happy to help. good luck.