from recollection, it might be 3/8 bsf head and so take an 18mm spanner quite well -
The Classic Mechanic: MM, AF, BSW, BSF Spanner Conversion Chart
however, be careful, because the plug you are trying to undo is aluminium and very easy to damage. before realising this, I tried to undo mine with an impact wrench, and ended up having to take off the whole unit...