13/16 is nice and tight.
Little new to metric. 21 mm fits - 19 is too small - don't know if there is a 20. Seems to go from 19 to 21.
Choice between 21 and 13/16 would be the 13/16 unless you wat to put something over it to protect - then the 21.
This is for the optional lug. I think the standard are the same.