SB 3.26.52

etad aṇḍaṁ viśeṣākhyaṁ
 krama-vṛddhair daśottaraiḥ
toyādibhiḥ parivṛtaṁ
 pradhānenāvṛtair bahiḥ
yatra loka-vitāno ’yaṁ
 rūpaṁ bhagavato hareḥ
Synonyms: 
etat — this; aṇḍam — egg; viśeṣa-ākhyam — called viśeṣa; krama — one after another; vṛddhaiḥ — increased; daśa — ten times; uttaraiḥ — greater; toya-ādibhiḥ — by water and so on; parivṛtam — enveloped; pradhānena — by pradhāna; āvṛtaiḥ — covered; bahiḥ — on the outside; yatra — where; loka-vitānaḥ — the extension of the planetary systems; ayam — this; rūpam — form; bhagavataḥ — of the Supreme Personality of Godhead; hareḥ — of Lord Hari.
Translation: 
This universal egg, or the universe in the shape of an egg, is called the manifestation of material energy. Its layers of water, air, fire, sky, ego and mahat-tattva increase in thickness one after another. Each layer is ten times bigger than the previous one, and the final outside layer is covered by pradhāna. Within this egg is the universal form of Lord Hari, of whose body the fourteen planetary systems are parts.
Purport: 

This universe, or the universal sky which we can visualize with its innumerable planets, is shaped just like an egg. As an egg is covered by a shell, the universe is also covered by various layers. The first layer is water, the next is fire, then air, then sky, and the ultimate holding crust is pradhāna. Within this egglike universe is the universal form of the Lord as the virāṭ-puruṣa. All the different planetary situations are parts of His body. This is already explained in the beginning of Śrīmad-Bhāgavatam, Second Canto. The planetary systems are considered to form different bodily parts of that universal form of the Lord. Persons who cannot directly engage in the worship of the transcendental form of the Lord are advised to think of and worship this universal form. The lowest planetary system, Pātāla, is considered to be the sole of the Supreme Lord, and the earth is considered to be the belly of the Lord. Brahmaloka, or the highest planetary system, where Brahmā lives, is considered to be the head of the Lord.

This virāṭ-puruṣa is considered an incarnation of the Lord. The original form of the Lord is Kṛṣṇa, as confirmed in Brahma-saṁhitā: ādi-puruṣa. The virāṭ-puruṣa is also puruṣa, but He is not ādi-puruṣa. The ādi-puruṣa is Kṛṣṇa: īśvaraḥ paramaḥ kṛṣṇaḥ sac-cid-ānanda-vigrahaḥ/ anādir ādir govindaḥ. In Bhagavad-gītā Kṛṣṇa is also accepted as the ādi-puruṣa, the original. Kṛṣṇa says, “No one is greater than Me.” There are innumerable expansions of the Lord, and all of them are puruṣas, or enjoyers, but neither the virāṭ-puruṣa nor the puruṣa-avatāras — Kāraṇodakaśāyī Viṣṇu, Garbhodakaśāyī Viṣṇu and Kṣīrodakaśāyī Viṣṇu — nor any of the many other expansions, is the original. In each universe there are Garbhodakaśāyī Viṣṇu, the virāṭ-puruṣa and Kṣīrodakaśāyī Viṣṇu. The active manifestation of the virāṭ-puruṣa is described here. Persons who are in the lower grade of understanding regarding the Supreme Personality of Godhead may think of the universal form of the Lord, for that is advised in the Bhāgavatam.

The dimensions of the universe are estimated here. The outer covering is made of layers of water, air, fire, sky, ego and mahat-tattva, and each layer is ten times greater than the one previous. The space within the hollow of the universe cannot be measured by any human scientist or anyone else, and beyond the hollow there are seven coverings, each one ten times greater than the one preceding it. The layer of water is ten times greater than the diameter of the universe, and the layer of fire is ten times greater than that of water. Similarly, the layer of air is ten times greater than that of fire. These dimensions are all inconceivable to the tiny brain of a human being.

It is also stated that this description is of only one egglike universe. There are innumerable universes besides this one, and some of them are many, many times greater. It is considered, in fact, that this universe is the smallest; therefore the predominating superintendent, or Brahmā, has only four heads for management. In other universes, which are far greater than this one, Brahmā has more heads. In the Caitanya-caritāmṛta it is stated that all these Brahmās were called one day by Lord Kṛṣṇa on the inquiry of the small Brahmā, who, after seeing all the larger Brahmās, was thunderstruck. That is the inconceivable potency of the Lord. No one can measure the length and breadth of God by speculation or by false identification with God. These attempts are symptoms of lunacy.