I believe 15mm is the correct size, I made my own by taking an old 9/16" open end wrench and grinding it thinner and using my 4 1/2' angle grinder to make the opening the correct size, for me that was a great re-purposing of the old wore out 9/16" wrench.
But yeah, I think 15mm is the right one for the standard bike wheels.
I'll measure mine for sure and post back here to confirm if 15mm is correct or not
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