The nut size should be the same as the spark plug socket. 22mm I think.
The allen key part is you need to hold the strut piston rod that goes through the centre of the nut. You can fit an allen key through the centre of the spark plug socket and then undo with a spanner on the flats of the spark plug socket. You are better off using a long allen key that fits into a socket to give you more leverage.
Instead of the spark plug socket you can use a deep swan neck ring spanner if you want.
Paul