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Or what size hexagon sizes would you use.
Well, if you don't have a specific size that you are looking for, I'd buy sizes that most nearly matched common imperial sizes (notwithstanding that US military equipment is supposed to be metric...)
For a good selection I'd pick 1/2", 3/4", 1" and 1.25" sizes. That would allow you to replicate a lot of the more common fasteners. Divide those numbers by 35 and that will give you a decimal size to try to match, eg, for a nut 1/2"across the flats, .5/35 = .014" so a resin bold or nut that is .014" across the flats will be the scale replica of a real bolt 1/2" across the flats.
Paul