All Credit goes to Dvorak1902 for the calc and Kiryu-MG3 for the manga scans[]
The Feat[]
As a desesperate move to win the fight Orga alterates his size and becomes stronger and bigger Hyper Orga, he proceeds to devour Godzilla 2000 but the king of the monsters charges his final Giga Blast and destroys Hyper Orga for good.
The Calc[]
We have the height of Hyper Orga, It's like a giant crocodile head with legs.
Then
Result: Low 7-B, Small City level
Megatons: Between 1.64 and 3.28 Megatons.