@Matt
I'm not sure where you're coming from. Unless you are suggesting that we disregard charcter statement and instead only ever consider Kubo's art I don't see the point of this. Going purely by statements: Seireitei is big (Twice the size of Texas?) - Gremmy's meteor could break it - Kenpachi casually overwhelmed the attack capable of destroying the Seireitei. Kenpachi casual Texas buster? Since, I think this is considered an outlier, and going purely by art caused the birth of many arguementative threads, a healthy balance of statements, and consistent artwork was needed.
Because Kubo's art is unreliable, (I believe everyone agreed to that on another thread I read) we must use statements, as long as those statements themselves are reliable (child Nel vs Yoruichi). Now, since the locations in question are still intact, and have never been directly targeted or struck by any attack that are stated to be capable of destroying them, the potential of those attacks are still in debate I believe, and this site generally only uses the capacity shown.
Now, unlike permanent structures/locations, explosions are fleeting and usually are only shown in, at most a small handful of panels, and their scopes are just about never mentioned by the characters, we are thus forced to use what we can visully see and so pixel scaling is the result.
As for the meteor, the size of the meteor is never stated
Also, I can't find an official meteor scan that shows both the meteor and the full Seireitei. Most panels show the meteor from the point of view of the ground, meaning we would need the size of the meteor to figure out how far they are from the meteor, and thus the relation between their size and the meteor's. And as the only information we are given on the meteor's size is how it pixel scales to the seireitei.
A thought; do we need the meteor's size? If the size is being so contested, why not simply find the force needed to destroy the seireitei (with different ends, surface wipe, pulverize, crater it idk) and just scale to that. Ignore the size entirely.