Offhand, I'm not sure of the size, I think its a 17mm, but you need a thin-wall socket. The one made by Motion-Pro is excellent, and easily fits in the recess around the spark plug. tommyturbo2
Or just trying bumming one from your dealership. When I was looking for spark plug socket for my Speedmaster, I ended up buying the thing right out of a mechanic's tool box at the dealership in Pensacola.
You will find that the centre plug in almost impossible to get to.You will need the OE tool from your dealer.This is a two piece tool,the socket and an allen key.