I personally prefer 3/32 air holes. For reference, I usually build my drippers in the .3-.5 range. I've tried 7/64, but find that just a touch too large for my taste. The resistance/wattage you vape at should heavily favor into your decision.
However, I would strongly recommend starting small. You can always make a hole larger, but you can't make them any smaller. It would be a shame to ruin a
device in haste. Start at 1/16th, vape it for a day or two, and go from there. If you find it too tight/hot, go up to 5/64th.. that's where I vaped for the longest time. If you still need more air, try 3/32nd.
Some advice.. use a good pair of vice grips with a thick rubber band too keep from scratching the
device. Lay it flat on a towel on top of your desk to drill. This should hold things steady and make it much easier to get a clean hole. I'd also recommend using a new carbon drill bit anytime your drilling a device. Use some
e-liquid to lube up your bit and you should be good to go.
I also like to finish airholes with a counter-sink bit. This is will give you a nice, clean chamfered edge which will look more natural and reduce any whistling sound. All it takes is a couple go's. Again, take it slow. Too much and you'll widen the airhole more than you've planned.
Best of luck.