What size spark plug socket do I need for my 1988 model Mercury Grand Marquis? on 1990 Mercury Grand Marquis

I've lost my owners manual and could not find the information in my haynes manual. I had trouble getting the spark plug out because none of the sockets I had would grip the spark plug. I used a 5/8 first then 13/16.

5/8 deep well. make sure it is clean around the plug or the socket will not go on.