I would prefer trunks of black locust. Branches can be used, but those are generally very gnarly and loaded with knots. Take as much as you can though; start with the trunks but take some branches as well if you like challenging character bows.
Diameter should be at least 3". I prefer 6 to 8" diameter though. Length should be over 6 feet, but I prefer 7 feet myself. That way, some small end cracks won't hurt, and if there's an unexpected knot in the wood, you may be able to lay out the bow in such a way that the knots ends up in the handle.
Since you don't use the sapwood in black locust, you'll lose some thickness. A sucker/starter of only 3" diameter may have only one inch of heartwood, which is not ideal.
Do NOT remove the bark from black locust, UNLESS you also remove the sapwood straight away. If you remove the bark, the sapwood will split terribly, creating cracks that go into the heartwood. This can make perfect staves unuseable.
Black locust is easy to identify. Google some pictures now, and see if it matches the trees you see in the field. If they're not black locust after all, you can still cut them. Maybe we can identify it as another good bowwood...