In all reality I don't think the actually thickness of the chains matter.Just as long as it provides the called for weight and deload at the bottom. I will say from my own set up with chains that using smaller diameter chains are a pain in the ass since you have to use more chains too make up the difference compared too the thicker ones.
I have to look it up. But I think the normal length is five. Dont forgett that you will also use a lighter secondary chain.
Here this will explain it better.