Twice the source mean radius; body diameter only, excluding rings.
Put Ganymede beside Mercury. The result is a reminder that “moon” describes a relationship, not a size category. You can compare an object’s diameter without assuming that its astronomical label predicts the result.
Follow the comparison
Next, put Ganymede beside Jupiter. A large moon becomes a small disk against a giant planet. The scene compares their bodies, not their orbital separation or gravitational influence.