CC Madhya 21.84

kṛṣṇa kahe, “ei brahmāṇḍa pañcāśat koṭi yojana
ati kṣudra, tāte tomāra cāri vadana
Word for word: 
kṛṣṇa kahe — Kṛṣṇa said; ei brahmāṇḍa — this universe; pañcāśat koṭi yojana — four billion miles; ati kṣudra — very small; tāte — therefore; tomāra — your; cāri vadana — four faces.
“Kṛṣṇa said, ‘Your particular universe has a diameter of four billion miles; therefore it is the smallest of all the universes. Consequently you have only four heads.

Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura, one of the greatest astrologers of his time, gives information from the Siddhānta-śiromaṇi that this universe measures 18,712,069,200,000,000 × 8 miles. This is the circumference of this universe. According to some, this is only half the circumference.