It's a 13mm. I used to check mine every couple months because I had the same thing happen at 75mph in a rainstorm. I removed the nut and reinstalled with a nylon locking nut to keep it from loosening. Don't go too tight so you don't stretch or break the stud.

I also use 20" wipers, they seem more stable and wipe better than the standard 22", which seems to flutter around and not wipe anything at highway speeds.