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Noah has been given a great offer by the world's core! He was offered the literal help of the planet itself for him to grow as strong as possible, so his Divine Realm could grow in size and fit an entire planet inside.
Although Noah's Divine Realm even had two moons, these moons size was not like that of the actual satellite, but around the size of mount Everest, if not a bit bigger each one.
The total size of Noah's Divine Realm would be around the entirety of north America, if not a bit of south America. There were large blood seas, which gave it a bit more size, but this was certainly not enough.
Also, there wasn't an immense outer space around it, the space was only open for a bit around the habitable soil and sea, after that, space "ended" and there was an invisible spatial wall in there.
Check "The Protagnist's Sister Is Actually the Strongest"!