It's either 4 or 4.5mm, something along that line.
There are actually three adjusters, the two you can get to easily and a third you have to take the assembly out to change. I could not get a good adjustment out of the box, I had to adjust the third one quite a bit, almost the length of the adjustment. By doing so, I got the full range of adjustment out of the two accessible screws and adjusted properly.
If that was hard to follow, look at it this way: It takes 3 points to establish aim. Up and down, as said, is the one sticking straight up. Left and right is the one you need a ratchet or pair of pliers to change. The third one is the one you can't easily get to. If you can't get a proper adjustment, then you need to take the assembly back out and adjust the third one, and don't be bashful, run at least half way through the range. Then mount it back up and try adjusting or fine tuning, if you will, with the 2 adjusters you can reach.
I know this would be better with pictures, it would make a half decent how-to, actually.