I'll assume you have the Kicker KX1200.1. The terminals accept 1/0 GA. If you're using that amp alone, I would run at least 2 GA from the battery. I'm using that amp along with the KX450.2. I'm running 1/0 GA to my dist. block. I'm running 1/0 GA from the dist. block to the KX1200.1 and 4 GA from the dist. block to the KX450.2.
That amp comes with a 150A fuse, so you need to use a power wire that can carry that current at the length you're running the wire.