The holy grail is lost for centuries ... but maybe it wouldn't be missed, if it were small enough to be used as a ring? Whatever, this is not the holy grail and not the tiniest espresso cup of the world: it's just a ring with a modest and clean design!
The ring size is US 6.5
Inside Diameter: 16.92 mm / 0.666 in
Inside circumference: 53.15 mm / 2.09 in