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Hey Edmund, are you Demonito from SF? Welcome to the site matey.
Best idea is to call him and he'll talk you through the process.
He'll need to know a shoulder measurement, sleeve length, chest measurement and back length etc... that you are comfortable with, possibly from an existing A-2 repro. He'll take some or all of the measurements and make it to fit you. Obviously some of the measurements will be dependant upon contract.
Think about how you want it to fit, do you want it longer than an A-2 you already own, wider in the chest etc.? And base your measurements around this.