As I’ve said before, some anime like to have things so big that the square/cube law says they shouldn’t be able to support their own weight. But weight only applies when there’s gravity, right? So how about giants in space?
Starting with a not-quite-anime example, Transformers: The Movie introduced Unicron, who transformed into a planet. Now, Cybertron’s exact size is unknown (like a lot of other things in Transformers), but in the cartoon it seemed to be smaller than Earth and maybe smaller than the moon, and Unicron is clearly shown to be smaller than Cybertron (his height as a robot is about equal to its diameter), but as a guess I’m going to say that his height is at least 2000 km and possibly considerably more. (The comic showed him to be the same size relative to Cybertron, and also never retconned away a line of narration that had claimed Cybertron was Saturn-sized.)
For an example with a much more clearly defined size, there’s the Cathedral Terra/Chouginga Dai-Gurren/Super Galaxy Dai-Gurren from Gurren Lagann. This spaceship is not only roughly the size of the moon, it’s actually a plot point that its mass is the same as the moon’s. It’s shaped a lot like a seafaring ship with sci-fi engines on it (and a face on the front); then of course it transforms into the third largest humanoid mecha in the series. (The ones that are larger are actually made out of physics-defying spiral energy and aren’t seen in the normal universe.)
There are other ships, mecha, or other giant solid structures of various shapes that are as big as planets. Diebuster. Giant naked Rei. Why are all the actual anime examples I can think of from Gainax?
Well, to top it off, there’s the Getter Emperor from the Getter Robo manga, which is continuously growing and gets at least as big as a galaxy.
Now, consider this. The definition of a planet has three criteria. The first one is that it orbits a star. (Technically, the IAU was shortsighted and said “orbits the sun” instead, but I’ve never seen any astronomer suggest that the numerous known exoplanets aren’t planets.) And the third is that it has to have “cleared its neighborhood”; this is the one that got Pluto kicked out of the club, and was apparently added to keep the number of planets a single-digit number instead of having to count Ceres, Makemake, etc. like the first proposed definition would have.
The second is that it has achieved hydrostatic equilibrium, which is basically a fancy way of saying it’s roughly spherical because its gravity is stronger than any forces that could maintain any different shape. Since gravity is directly proportional to mass, this is effectively what defines a minimum size rule for planets.
But what I’m getting at is that it also relates to a maximum size rule for anything with irregular shapes; there is a certain mass above which any discrete object of significant density can’t be any shape other than roughly spherical. And all of the examples that I’ve named are well above the sizes of Ceres (the largest object in the asteroid belt) and Charon (Pluto’s largest moon), which are both definitely spherical. How do they maintain their very non-spherical shapes? Transforming at that size is even harder. (Of course, spiral power does explicitly break physics.)