4mm. It's a metric size, but if you insist on standard, 5/32" should work.
And yes, there are special tools for that. The one I am using is a slim double-sided 4/4.5mm 'gear wrench'. You can find those on line from many sources (just google 'Ford headlight adjuster'). I think that I got mine from JEGS for about $15). These are are designed to fit in tight spaces, but on the Ex there is plenty of room, so a 4mm socket on a 1/4" ratchet drive should work.