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

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.