Full Idea
Democritus say that it is possible that there exists an atom of the size of the world.
Gist of Idea
There could be an atom the size of the world
Source
report of Democritus (fragments/reports [c.431 BCE], A047) by Pseudo-Plutarch - On the Doctrine of the Philosophers 1.12.6
A Reaction
The editor says this may have been a criticism of Democritus - presumably a reductio ad absurdum. But Democritus has no upper limit on the size of an atom. It challenges the imagination to treat such a huge thing as indivisible.
Book Reference
Democritus: 'Early Greek Phil VII: Democritus', ed/tr. Laks,A/Most,G [Harvard Loeb 2016], p.117