I'm not familiar enough with the math to figure it out, but the footnote for the image mentions the photo is from 21 miles above, and wikipedia [1] reports the cameras focal length is 150mm. Is it possible to calculate the relative scale with that?

[1] https://en.wikipedia.org/wiki/Dawn_(spacecraft)

You can calculate the field of view from the focal length, but you need to also know the film/sensor size.

However the Wiki article you linked to said the FOV of the framing camera is 5.5° x 5.5°.

So at 21 miles, the photos would be about (5.5°/360° ⋅ 2𝛑 ⋅ 21 miles) = 2.0 miles across.

That’s 2 miles across each, right? The image in the blog is a montage of images, so now to figure that out...