What acoustic guitar string gauge should I use for rhythm playing in a folk-rock band?

Go with 12-53 as a safe bet. It gives you enough tension for volume in a band mix while still letting you bend strings — heavier strings (13-56) project better but fight you on bends, and lighter (11-52) bend easy but get swallowed by a drummer. If you’re tuning down a half-step or full step, drop to 11-52 to keep the same feel.