Are we referring to the nut that holds the upright rod to the bumper? The threads visible in the photo below? My guess for that would be 6mm. The hardware that secures the circular reflector to the upright may be smaller.
Thanks guys - I was originally just looking for the nut - but now I need to get the additional stuff Osso refers to - which make sense - and probably why my nut fell off originally (best I fix both sides before I lose the other).