Twice the source mean radius; body diameter only, excluding rings.
Mars is an inviting step away from Earth in this collection: smaller, but still large enough for the difference to be immediately visible. Compare the disks before reading the ratio and see how close your first impression comes.
A diameter is not a volume
A world roughly half as wide has much less than half the volume. Use the spherical-volume result below the scene to explore that distinction. Neither number measures mass or tells you how many solid spheres could be packed inside another.