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.
the ones I use were cut so each length weighs about ten lbs. This makes it very easy to compute. Use cable loops and carbiners to attach them. If you are pulling or bencing the carbiner usually goes in the middle of the links. If squatting at the end. does that make sense?