What spark plug socket for a mcculloch 210 strimmer?

How do I find out what size socket I need to remove the spark plug on my trimmac 210? I have a 5/8" socket but its too small and I'm not sure where to measure. The spark plug is an RCJ6Y, please help.
stuck, May 2005


done
19mm box spanner i use

tomo, August 2009
I 19mm ring spanner worked well for this

Graham, June 2005
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