If you have a set of vernier calipers you can measure what size hole you need but just measuring some of mine I would use something between a 1.5 (maybe a little tight) to at most 1.8 diameter drill but I have found that the screws are cheap garbage that are soft and the slot strips easy and I always replace them. Another thing I have found and was told by my Dad was, as he used to say, when putting screws into wood just use some mouth lube (spit) but I upgraded to a more modern and less harmful lube and just use the smallest wipe of Vaseline and the screws go in much easier and it doesn't introduce moisture and have never broken a screw. If you need to get them out they don't rust in the hole like with spit and have never had a screw come out and you can use a slightly smaller hole so it holds better. Don't forget to use a bit of masking tape as a depth guide so you don't drill through the head.