Variation on style and size?

I have a product that comes with either a plastic fitting or metal fitting. The fittings are slightly different sizes causing 1 product to be about 3/4″ difference in size.

Would I still use “style” as the variation or would I use “size” and instead of listing the sizes just put the name of the style?

Also, I already created both listings so what would the best option to convert the ASINs to parent/child? This is my first time doing this.