Spark Plug tool help

I’m after a spark plug tool to put in a ngk gr9di-8 for my Rotax max. However I can’t seem to find which size I need. Is it the 16mm or 21mm sprocket size?

Please answer in metric as I would like to avoid confusion.

Thank you

I’ve only used two sizes of spark plug wrenches. Both are deep well sockets with a rubber insert. One is 13/16" and the other is 5/8". 13/16" converts to 20.6 mm and 5/8" converts to 15.9. If you purchase both, you should be able to work and any small gas engine. My guess would be the 13/16.
None of the web sites in the US list a GR9DI plug.

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21mm socket or 21mm long nose plug socket

great thank you. I purchased a two in one tool 16+21mm. worked great

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Yep was going to say 16mm and 21mm are what you’re looking for.