Twice the source mean radius; body diameter only, excluding rings.
The Moon and Earth are a natural starting pair. Their scale difference is large enough to notice immediately, yet small enough that neither disk disappears.
Size without distance
The gap in this scene is chosen to make the outlines readable. It is not the Earth–Moon distance. A drawing that represented both diameter and orbital separation at once would need a very different layout.
Open Earth versus the Moon, then turn Earth into a marble to find the matching miniature Moon.