What size reducing washer do I need?
They do make reducing washers if your original hole is ~1-1/8 it is 1” using 12 awg Romex will take a 1/2” clamp down you can use 2 1’ to 1/2 “ washers one outside one inside the box or plug that hole and find an unused 1/2” hole , I don’t remember for sure but think a large blue or blue gray wirenut will cover a  single #6 wire , I would not cut them in case they are wanted at a later time.