Bows in the 60-66" range can be just as durable as the 66-72" range. The trick is knowing how wide to make the bow for the specific type of wood you will be using. This "trick" is covered in TBB. As a general rule, the less set a bow takes, the longer it will last.
In my opinion, osage and hickory are the two best woods you can use to begin building bows. If you look hard enough you don't have to spend a whole lot of money on hickory or osage staves. A lot of guys are even willing to trade something for it.
Sometimes you can find branches or saplings of these woods (for free or very cheap) that will make fine bows.
If you want to gain some experience before working the "good stuff" I don't recommend lumberyard oak. I think ash is a better choice....it's slightly less brittle. The best wood for experimenting (in my view) comes from tree trimmings, fallen trees, and bulldozed tress. Make sure the wood is insect free and green.
Another important thing to keep in mind is the level of humidity the bow will be exposed to. Hickory is good in dry climates and osage is good in humid climates.
