OK, Wb,
The blade of a screwdriver that fits the hole in the connecter is called a 3/32" (flat blade) screwdriver. It Calipers at 0.102 ", and is just about the largest that one could use.
So, would guess that a 2.5 MM would be the largest one might want to use.
To me, this Aux connector is out in the open, and an un-insulated screwdriver should be fine for almost any of us to manipulate in the area, without risk.
Good that you are asking, as many off-grid, rural sites do not have a sufficiently small flat blade screwdriver for this task.
Good Luck, Vic