How big a hole do I need to cut behind the tub faucet to replace it? Want plenty of working room.

The valves are 24" above the floor, the spout about 19". 3 valve type with 8" spacing.

the big question is: How much above & below that should I allow to get to the unions, if present? I want to cut a single piece of drywall out so I can replace it easily.

Its in a closet so I can go large... stud indicator says there's a stud in the middle.