IME/IMO it depends on the screws you will use and also the neck shaft wood type
to determine the drill size, use a dial caliper on the screw shaft (the solid shaft area at the center of the threads, not the non-threaded area near the screw head) to determine its diameter. this is the generic hole size your screw shaft must fit into. once you have this you then give consideration to the diameter of the screw threads, how deep you are inserting the screw into the neck shaft, and also the Jenka hardness of the wood you're screwing into
if you have an ebony neck shaft, you will want some added hole diameter to offset for the hardness of the wood you're threading into. if instead you're threading into a softer wood like Red Alder (not that you would use Alder for a neck), having a hole diameter that is the same or even slightly smaller will help to solidify the threads as they cut into the wood
if all else fails, simply using the screw shaft diameter will work
all the best,
R