Twice the source mean radius; body diameter only, excluding rings.
Ceres starts our guided journey. Beside the Moon it has a substantial outline; beside a giant star it becomes too small to distinguish at this scale. The physical object has not changed—only its context has.
When a world becomes a dot
If an object is too small to see clearly, the scene adds a locator around its true position. The locator is not an enlarged measurement. The numerical ratio remains available, and the marble analogy gives another way to understand the difference.