Good question. The pilot hole should be the diameter (or slightly larger) of the root of the screw you're going to use. The best way to measure the root of a screw is with calipers because they can get between the threads. Or, simply hold the screw in front or in back of a drill and eyeball it up. In other words, guess at it. It also depends on what kind of wood it is. The harder the wood, the bigger the drilled hole must be. Basically, when the screw enters the wood, you only want the threads biting into the wood, not the shank of the screw.
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