You should compare the diameter of the copper conductor with a conductor of a known size. It may be #12. I doubt that someone would run #10 to receptacle branch circuits. If it is smaller than #12 it is probably #14.
Fuses should be not larger than the following sizes:
15 Amp for #14; 20 Amp for #12; and 30 Amp for #10 wire.
You should also check at the subpanel to verify that the fuse is correct for the wire size going from the subpanel. That is the place that I would start.