For the speaker wires themselves 18-22 gauge should be fine even smaller will work ok...I like 18-22 to ensure good power flow. If the amp is feeding a sub 18 is my choice here.
For amp power leads for an amp that small you can likely run 8 or even 10 gauge wire and be fine. Check what the amp manufacturer suggests. The size of the hookups for power on the amp can also be a clue, small hookup spots generally mean smaller wire.
You should run the same size power and ground leads.