It can be a size bigger or smaller than other wires. The bigger ground wire doesn't do any harm to the system. It'll only cost more to use a bigger ground wire. You can consider using a junction box to extend the wire.

What happens when the ground wire is too small?

With too-small a ground wire, the breaker cannot magnetic-trip because it cannot flow enough current to hit 10x, because of the too-high resistance of the wire.

Does the length of a ground wire matter?

Yes, any length is OK. Keep in mind that a TT ground has nothing to do with the sound as it is not part of the audio path. As long as electrons have a path to a proper ground, you are good.