Inspired by this answer:
Suppose there is an enormous dragon sleeping underground (let's not worry about the biomechanics of how such a creature could exist). One day, it wakes up, shakes the earth off its back, and takes off, leaving the planet and flying into space, to destinations unknown. In the linked answer, @elemtilas demonstrated that a dragon the size of Asia would destroy the planet, returning it to a ball of molten rock and stripping most of the atmosphere. My question is, how large could such a dragon conceivably be, to be able to take off without eradicating all life on the planet?
EDIT: Okay, let's get some devastation scale definitions. Using this classification of end of the world scenarios, the dragon in the previous answer caused a class 5 event, close enough. What would be the largest dragon that could take off while staying below, say, a class 4? Biosphere potentially damaged, but not destroyed, and wide-scale diverse life will recover and continue on Earth.
(By request, the listed classes from that site)
0: Regional Catastrophe (examples: moderate-case global warming, minor asteroid impact, local thermonuclear war) Global civilization not eliminated, but regional civilizations effectively destroyed; millions to hundreds of millions dead, but large parts of humankind retain current social and technological conditions. Chance of humankind recovery: excellent. Species local to the catastrophe likely die off, and post-catastrophe effects (refugees, fallout, etc.) may kill more. Chance of biosphere recovery: excellent.
1: Human Die-Back (examples: extreme-case global warming, moderate asteroid impact, global thermonuclear war) Global civilization set back to pre- or low-industrial conditions; several billion or more dead, but human species as a whole survives, in pockets of varying technological and social conditions. Chance of humankind recovery: moderate. Most non-human species on brink of extinction die off, but most other plant and animal species remain and, eventually, flourish. Chance of biosphere recovery: excellent.
2: Civilization Extinction (examples: worst-case global warming, significant asteroid impact, early-era molecular nanotech warfare) Global civilization destroyed; millions (at most) remain alive, in isolated locations, with ongoing death rate likely exceeding birth rate. Chance of humankind recovery: slim. Many non-human species die off, but some remain and, over time, begin to expand and diverge. Chance of biosphere recovery: good.
3a: Human Extinction-Engineered (examples: targeted nano-plague, engineered sterility absent radical life extension) Global civilization destroyed; all humans dead. Conditions triggering this are human-specific, so other species are, for the most part, unaffected. Chance of humankind recovery: nil. Chance of biosphere recovery: excellent. 3b: Human Extinction-Natural (examples: major asteroid impact, methane clathrates melt) Global civilization destroyed; all humans dead. Conditions triggering this are general and global, so other species are greatly affected, as well. Chance of humankind recovery: nil. Chance of biosphere recovery: moderate.
4: Biosphere Extinction (examples: massive asteroid impact, "iceball Earth" reemergence, late-era molecular nanotech warfare) Global civilization destroyed; all humans dead. Biosphere massively disrupted, with the wholesale elimination of many niches. Chance of humankind recovery: nil. Chance of biosphere recovery: slim. Chance of eventual re-emergence of organic life: good.
5: Planetary Extinction (examples: dwarf-planet-scale asteroid impact, nearby gamma-ray burst) Global civilization destroyed; all humans dead. Biosphere effectively destroyed; all species extinct. Geophysical disruption sufficient to prevent or greatly hinder re-emergence of organic life.
X: Planetary Elimination (example: post-Singularity beings disassemble planet to make computronium) Global civilization destroyed; all humans dead. Ecosystem destroyed; all species extinct. Planet itself destroyed.