The size of wire that you need for 60 amps is a #6 or 6 AWG copper wire.
The national electrical code states that a #6 AWG copper wire is required for 60 amp circuits.
When using aluminum wire you need a #4 AWG wire as aluminum wire needs to be larger to carry the same amount of amperage that a smaller copper wire can handle.
Copper wire should be used indoors and aluminum wire can be used outside such as for your service entrance wire.
But indoors you need to use copper wire.
Copper wire is much better at conducting electricity as well and less prone to failure.
Aluminum wire can corrode and even come apart more easily and cause loose connections more than copper wire can.
This is why aluminum wire is not used anymore in electrical circuits in homes.
Copper wire is always now used inside homes other than the service entrance wire.