I think you will find it is a 14mm size,but you need a double hexagon deep thin wall socket..Most manufacturers are now using this type of spark plug.Special socket is available from ECP car parts or any good motor factor.hth

You can buy the correct spark plug sockets at most good accessory stores. They have a runbber insert in to prevent the plug falling out.

Here is a tip I picked up some years ago. Never start screwing a plug into an alloy head using a socket. Its very easy to cross thread them and the resulting damage is expensive to fix. Get a piece of 8mm fuel pipe about 8" long and push it onto the plug, inset down into the head and start to screw, if its cross threaded the rubber will simply slip on the plug with no damage, otherwise it will screw the plug in correctly. Tighten with torque wrench to correct setting, again alloy is easy to damage and expensive to repair.

Always put a bit of copperslip on the threas as well, many Ford engines with 40,000 mile plug changes have had the plus sieze in.