14 Guage is good enough but for lenght it depends on the distance you are putting the speaker from the amp. It's entirely up to you.
You always keep your wires as short as possible because of attenuation. The longer the cable , the weaker the signal becomes, as far as gauge 12 guage works perfect in most applications 10 for bass if possible, and make sure its a good quality copper wire. Not all wire is equal.
