Twice the source mean radius; body diameter only, excluding rings.
Neptune and Uranus make a useful pairing when you want to examine small differences rather than enormous ones. Their disks are close in size, so the numerical result helps where a quick glance may not.
What the illustration means
The artwork identifies the object; it is not a scientific surface map. The disk’s diameter carries the measurement. Use the same mean-diameter convention for both planets before comparing their outlines.