I like 10-14" for ease of handling as well as a flatter back.
Look for at least 72" of clear (no knots or limbs and straightish) wood if you are thinking staves or 40" if billets.
Then look at the bark and see if it twists or barber poles any.
When you find one that meets these tests, cut it as close to the ground as possible. Osage hangs up bad because it is knarly and strong. A clear drop path is best. Failing that, a come-along or better yet an ATV or truck and strong strap or chain.
Cut it to length and get as much wood as you can, seeking out 36" plus billets and 60" plus staves in all the main log and branches.
If you have to carry it, quarter it using wedges and sledges. here's ome wood my son and I cut a couple years ago:
Wood 2009 As soon as you can, coat the ends and 6" or so with shellac or latex paint to prevent checking. Stack it in a garage and get the bark and sapwood off at the same time. If you don't have indoor space, spray it with insecticide and keep it in the shade. I remove all the bark and sapwod from each stave at the same sitting and work through the pile until it is all processed and stored safely inside. The bugs will start eating in the spring and will be an inch or so deep within a year.
Wood dries at about an inch a year so the thinner you can get it, the quicker you can turn it into a bow.
Have fun. Wear ear and eye protection. Plan ahead and be careful