Take a piece of paper, wrap it around your wrist, mark the overlap, cut it to shape, use the measurement tool on your phone![]()
Yes, even easier, use a tape or garment measure and mark the inch overlap then convert to mm.
However, the question was toward the Ocean Band which is an all-in-one band with no specific size and at least one reviewer with small wrists (iJustine) felt it was too big.