Hi Lambert. I just found out the answer today from my local DIY store. They are M5.
I wanted to know the size as I've been trying to find a neat way to attach my apeman camera to the bike. This is a great tip. I drilled a 6mm hole in one of the supplied Apeman brackets, removed the bottom screw from the screen, and then replaced it by screwing in the apeman attachment using a new M5 screw. This means I can simply remove the camera and bracket from the screen when not needed.