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You can also call a propeller repair shop for the nut size, as they will have the nut on a shelf and can measure it.
Whether or not you end up using a ring spanner (I suspect that is what we Yanks call a "box wrench") or something else, like a pipe wrench (which I recommend,) it should be at least 18" long. It will be difficult to properly torque the nuts with anything shorter.