The fitting sizes on the compressor, condenser, evaporator and drier are going to dictate the hose sizes. You can download the instructions for whatever kit you are buying on the VA website and see what it says. Mike is right on and I have installed 3 VA systems that agreed with his numbers. The no. 6 might be all hardline depending on the kit you use but if you are doing a custom installation you will likely end up needing no. 6 hose too.
BTW, I just noticed you were looking for an AC expert which pretty much rules me out... :>)
Don
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