3/4
most likely it will be a 5/8" spark plug socket.
18MM
Mine is 3/4
5/8", available at Walmart $4.50 for a set of 3 spark plug sockets.
a standard 5/8 deep socket spark plug wrench
5/8 inch spark plug socket.
5/8 deep insulated spark plug socket
11/16inch or 18mm socket.
5/8 inch
5/8 plug socket
Most commonly a 13/16", but a 3/4" is possible.