I figured it would be no big deal to find the screw size for you, boy was I wrong! I went to the Yamaha parts section and pulled the headlight assy, I found the screws, but all they supply you with is their part number, not the size of the screw itself. I then tore through my repair manual, this time I found part numbers for everything but the screws? I tried for you but came up empty handed, sorry.
The same thing happened to me when I blew a fuse and started looking for the cause, I had to remove two of the panels in order to get a decent view of what was going on, when I started to take them apart I not only lost the screw but when I tried to remove the headlight adjuster from the panel the plastic piece that goes between the adjust cable and knob, that broke too(actually I believe it was already broke, all I did was try to take the nut off but it kept spinning, the plastic shaft itself was split). I ordered the plastic piece from Yamaha for a couple of bucks, the screw I had in my spare screw bin which isn't marked either. After I lost the first screw I used a magnetic phillips head screwdriver, the screw tried to fall when I put it back together, but the magnetic driver held it steady for me.
If I had the bike here I'd take one out and find the size for you, but the bike is in the shop right now.
As these screws are a pain in the buttox, I'm going to get several more for a rainy day. Good Luck and Ride Safe. C1