What size allen wrench do you use to change the rotors on a 1991 chevy s10?

There's no single Allen wrench size used to remove all brake rotors on a 1991 Chevy S10. The caliper mounting bolts are typically a 1/2-inch or 9/16-inch bolt head size, which is not an Allen wrench. They are usually sockets. You'll need a wrench or socket of that size, not an Allen key.

The rotor itself *might* have some Allen-head bolts or screws holding it to the hub, but this is less common. If it does, you'll need to check the rotor itself or consult a repair manual specific to your 1991 Chevy S10 to determine the size. It's possible no Allen wrench will be needed at all.