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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.

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    $\begingroup$ "without eradicating all life on the planet" and "leave at least some civilisation intact after it left" are two different questions. $\endgroup$
    – L.Dutch ♦
    Jul 19, 2019 at 10:56
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    $\begingroup$ "Flying off into space" implies a means of propulsion other than ordinary flight, so what is that ? Some types of propulsion could be devastating in their own right. $\endgroup$
    – StephenG - Help Ukraine
    Jul 19, 2019 at 12:04
  • $\begingroup$ That's actually a question of what is maximum mass of fuel for liftoff that wouldn't deny pull from "engine". $\endgroup$
    – SZCZERZO KŁY
    Jul 19, 2019 at 12:33
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    $\begingroup$ @StephenG hand-wavey, honestly. Assume the dragon can fly in space because it's a magical space dragon. $\endgroup$
    – Tam Coton
    Jul 19, 2019 at 13:49
  • $\begingroup$ First, thanks for taking the time to ask this query! Second, whenever questions come up that involve "all life" on a planet, there will invariably be much discussion & consternation over what that actually entails. Do you mean a single tiny colony of lithophages survives? For example, I left open the possibility of life surviving the Great Dragon's flight. Surely some extremophile life will survive in some antipodean sludge, perhaps learn how to eat molten magma or perhaps survive in one of those hill sized chunks of earth biding their time in low orbit. $\endgroup$
    – elemtilas
    Jul 19, 2019 at 15:16

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Working backwards from your specified end results, a class 3 event

First we'll assume this is not a man-made dragon, so it's a class 3b, not 3a.

In the examples of a 3b event, as cited in your reference site (and transcript), one item listed is a "major asteroid impact", and while reading the cited answer (especially phase V) I was reminded heavily of what I would imagine a "dwarf-planet-scale asteroid impact" might look like, which happens to be another citation from your reference site, matching the class 5 event you mentioned as well. And the dragon itself, in that answer, could reasonably be considered to be "dwarf-planet-scale".

So, I suggest treating this new dragon as a "major asteroid" scale, in terms of destruction.

The dwarf planet dragon was probably on the lower end of the spectrum, as far as mass and speed are concerned, for dwarf planet impactors go. But it made up for it by concentrating its force on 4 small points (each foot), poking narrower and deeper holes and releasing energy from VERY deep in the Earth.

However, since the major asteroid scale dragon won't be poking it's feet in to the mantel, it will have to increase its devastation capability, either by being on the higher end of the size spectrum for asteroid impacts, or by taking off at a speed that is faster than average for asteroid impacts, or some combination of the two.

How big of an asteroid is that? I suggest using the Chicxulub asteroid (the one cited to have caused the mass extinction of dinosaurs about 65 million years ago) as a reference. It's estimated to have been between 11 and 81 km in diameter.

Since asteroids are generally more spherical than dragons, we'll take this 80-ish km long asteroid (we're going for the large end of the spectrum, remember) and stretch it to Dragon-shape. Depending on the specific body style of this dragon (long thin eastern asia versions vs shorter stockier european, etc.) this will make it well in excess of 250 km long, maybe reaching 500 km or more. Wingspan will be even greater, say, 500 km to as much as 2500 km

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