People experienced with planet/planetoid calcs: How do you determine the diameter of a wobbly sphere?

[First_Witch]

Im stumped by this particular feat here and dont know how to proceed. I'm trying to find out how big the last planetoid in the last panel is and tried to do some pixelscaling with the spear in the third and fourth pannel, whose size I do know (3,82 meters). Due to the perspective and already caused damage, overlapping a clean circle was not possible for me. Any help is appreciated.

 

[IDK3465]

If I were you, I’d just give it my best guess as to where the actual diameter is in the 4th panel, scale it from the spear, and then scale the rest of the planetoids in the 5th panel.