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.
Translation: 
“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.
Purport: 

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.