Should be 17mm, I just changed mine last fall and have one in my stock tool kit....what do you want me to do with it?
It's thin-wall, on a KLR650 I haven't found a regular 18mm* socket that I could get down in the head. My option is to find an extra somebody has, grind down a good socket, or buy one online.
If you have an extra that you don't mind parting with, bring it along sometime that we'll be at the same ride. I'm not in a rush; I just bought a new NKG Iridium plug when I rebuilt the engine and I figured I might as well swap it out for the one in there.