If its the original nut it'll be 21mm, which is the same size as a large sparkplug socket. these also have spanner flats on so you can drop an allen key down the middle. if the nuts have been replaced they'll be 22mm, which is the same size as a lambda probe/diesel injector socket. these also have spanner flats on, and often have a slot cut into the side where you can get your allen key in.
You may struggle to fget the nuts off if they have been buzzed on by a garage though, worst case pop to a local garage and ask em nicely to buzz em loose then do up finger tight, then drive home and swap the suspension over
