6-8mm is fine, depending on what wires you'll be feeding through it. Get a scrap piece of wood and drill a test hole then feed all the wires you think you'll need through it (pickups, shield wire etc) and that'll tell you.
For a hardtail you can go from under the bridge to the control cavity or, for a shorter path, to the bridge pup rout. A long 3mm bit works well. To the pickup rout is safer if you're concerned (I did it here).
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