8"-10", as straight as possible as Osage is great wood, but a pain to work with. Stay away from those with visible spiral in the bark as that is likely to indicate twisted grain. Again, there are guys who can make it work, but they've already earned it by starting out with something easier most likely. Those are some of the outside indicators. Once you get inside is when it really gets good.
Check out the growth ring thickness once you get it cut. Thicker rings are easier to follow. Also, there will be dark and light layers to each ring. The light color is "early wood" and is dang near worthless. It's actually awful, even though osage is excellent wood. There needs to be a preponderance of dark, or "late wood" throughout. It's really the collective late wood of osage that makes it such good bow wood. I forget exactly where it was said, but it was mentioned, I think in one of the TBBs, that something like 5% of the total mass of osage alive is capable of producing a good bow. That may be an exageration, but the point is made. It's darn good wood, but darn tough to find some that will work and then darn tough to work with when you find it.
I was out earlier today kicking around my temporary home (temporary duty station) in southwest Oklahoma. I found some osage, but it was sure short, twisted, and rough looking. I think some of the mesquite around would be easier to work, and it would be tough.
Hop over to the primitive archer forum and ask some of those guys. They are magicians over there. Some of them really get into osage. I have yet to get that crazy about it, for all those above reasons.