SB 5.16.4

ṛṣir uvāca
na vai mahārāja bhagavato māyā-guṇa-vibhūteḥ kāṣṭhāṁ manasā vacasā vādhigantum alaṁ vibudhāyuṣāpi puruṣas tasmāt prādhān-yenaiva bhū-golaka-viśeṣaṁ nāma-rūpa-māna-lakṣaṇato vyākhyāsyāmaḥ.
Synonyms: 
ṛṣiḥ uvāca — Śrī Śukadeva Gosvāmī continued to speak; na — not; vai — indeed; mahā-rāja — O great King; bhagavataḥ — of the Supreme Personality of Godhead; māyā-guṇa-vibhūteḥ — of the transformation of the qualities of the material energy; kāṣṭhām — the end; manasā — by the mind; vacasā — by words; — either; adhigantum — to understand fully; alam — capable; vibudha-āyuṣā — with a duration of life like that of Brahmā; api — even; puruṣaḥ — a person; tasmāt — therefore; prādhānyena — by a general description of the chief places; eva — certainly; bhū-golaka-viśeṣam — the particular description of Bhūloka; nāma-rūpa — names and forms; māna — measurements; lakṣaṇataḥ — according to symptoms; vyākhyāsyāmaḥ — I shall try to explain.
Translation: 
The great ṛṣi Śukadeva Gosvāmī said: My dear King, there is no limit to the expansion of the Supreme Personality of Godhead’s material energy. This material world is a transformation of the material qualities [sattva-guṇa, rajo-guṇa and tamo-guṇa], yet no one could possibly explain it perfectly, even in a lifetime as long as that of Brahmā. No one in the material world is perfect, and an imperfect person could not describe this material universe accurately, even after continued speculation. O King, I shall nevertheless try to explain to you the principal regions, such as Bhūloka, with their names, forms, measurements and various symptoms.
Purport: 

The material world is only one fourth of the Supreme Personality of Godhead’s creation, but it is unlimited and impossible for anyone to know or describe, even with the qualification of a life as long as that of Brahmā, who lives for millions and millions of years. Modern scientists and astronomers try to explain the cosmic situation and the vastness of space, and some of them believe that all the glittering stars are different suns. From Bhagavad-gītā, however, we understand that all these stars (nakṣatras) are like the moon, in that they reflect the sunshine. They are not independent luminaries. Bhūloka is explained to be that portion of outer space through which the heat and light of the sun extend. Therefore it is natural to conclude that this universe extends in space as far as we can see and encompasses the glittering stars. Śrīla Śukadeva Gosvāmī admitted that to give full details of this expansive material universe would be impossible, but nevertheless he wanted to give the King as much knowledge as he had received through the paramparā system. We should conclude that if one cannot comprehend the material expansions of the Supreme Personality of Godhead, one certainly cannot estimate the expansiveness of the spiritual world. The Brahma-saṁhitā (5.33) confirms this:

advaitam acyutam anādim ananta-rūpam
 ādyaṁ purāṇa-puruṣaṁ nava-yauvanaṁ ca

The limits of the expansions of Govinda, the Supreme Personality of Godhead, cannot be estimated by anyone, even a person as perfect as Brahmā, not to speak of tiny scientists whose senses and instruments are all imperfect and who cannot give us information of even this one universe. We should therefore be satisfied with the information obtainable from Vedic sources as spoken by authorities like Śukadeva Gosvāmī.