CC Ādi 5.119

pañcāśat-koṭi-yojana pṛthivī-vistāra
yāṅra eka-phaṇe rahe sarṣapa-ākāra
Word for word: 
pañcāśat — fifty; koṭi — ten millions; yojana — eight miles; pṛthivī — of the universe; vistāra — breadth; yāṅra — whose; eka-phaṇe — on one of the hoods; rahe — stays; sarṣapa-ākāra — like a mustard seed.
Translation: 
The universe, which measures five hundred million yojanas in diameter, rests on one of His hoods like a mustard seed.
Purport: 

The Lord of Śvetadvīpa expands Himself as Śeṣa Nāga, who sustains all the planets upon His innumerable hoods. These huge global spheres are compared to grains of mustard resting on the spiritual hoods of Śeṣa Nāga. The scientists’ law of gravity is a partial explanation of Lord Saṅkarṣaṇa’s energy. The name “Saṅkarṣaṇa” has an etymological relationship to the idea of gravity. There is a reference to Śeṣa Nāga in Śrīmad-Bhāgavatam (5.17.21), where it is said:

yam āhur asya sthiti janma-saṁyamaṁ
tribhir vihīnaṁ yam anantam ṝṣayaḥ
na veda siddhārtham iva kvacit sthitaṁ
bhū-maṇḍalaṁ mūrdha-sahasra-dhāmasu

“O my Lord, the hymns of the Vedas proclaim that You are the effective cause for the creation, maintenance and destruction. But in fact You are transcendental to all limitations and are therefore known as unlimited. On Your thousands of hoods rest the innumerable global spheres, like grains of mustard so insignificant that You have no perception of their weight.” The Bhāgavatam further says (5.25.2):

yasyedaṁ kṣiti-maṇḍalaṁ bhagavato ’nanta-mūrteḥ sahasra-śirasa ekasminn eva śīrṣaṇi dhriyamāṇaṁ siddhārtha iva lakṣyate.

“Lord Anantadeva has thousands of hoods. Each sustains a global sphere that appears like a grain of mustard.”