Twice the source mean radius; body diameter only, excluding rings.
Mercury makes a useful challenge to our intuition: the label “planet” does not tell you how big something must be. Compare it with Ganymede and watch the moon become the larger disk.
Try a different neighbour
Choose Earth to give Mercury a familiar reference. Then switch to the Moon. The comparison concerns physical diameter only; it does not compare mass, surface gravity, or the distances between these worlds.