“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 pradhana. Within this egglike universe is the universal form of the Lord as the virat-purusa. All the different planetary situations are parts of His body. This is already explained in the beginning of Srimad-Bhagavatam, 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, Patala, 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 Brahma lives, is considered to be the head of the Lord.
This virat-purusa is considered an incarnation of the Lord. The original form of the Lord is Kṛṣṇa, as confirmed in Brahma-samhita: adi-purusa. The virat-purusa is also purusa, but He is not adi-purusa. The adi-purusa is Kṛṣṇa. Isvarah paramah Kṛṣṇah sac-cid-ananda-vigrahah. anadir adir govindah. In Bhagavadgita Kṛṣṇa is also accepted as the adi-purusa, the original. Kṛṣṇa says, “”No one is greater than I.”” There are innumerable expansions of the Lord, and all of them are purusas, or enjoyers, but neither the virat-purusa nor the purusa-avataras–Karanodakasayi Viṣṇu, Garbhodakasayi Viṣṇu and Ksirodakasayi Viṣṇu–nor any of the many other expansions, is the original. In each universe there are Garbhodakasayi Viṣṇu, the viratpurusa and Ksirodakasayi Viṣṇu. The active manifestation of the virat-purusa 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 Bhagavatam.
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 Brahma, has only four heads for management. In other universes, which are far greater than this one, Brahma has more heads. In the Caitanya-caritamrta it is stated that all these Brahmas were called one day by Lord Kṛṣṇa on the inquiry of the small Brahma, who, after seeing all the larger Brahmas, 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”
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Third Canto, Chapter 26 – Text 52