Srila Prabhupada - "The planets are called "dvīpas", just like there are islands in the watery ocean, these planets are in the ocean of outer space." (CC Madhya 20.218, Purport)

The massive material universes (Brahmandas) coming from Maha-Visnu each have a smaller secondary universes deep inside them.

These secondary universes within each Brahmanda universe, are created by Maha-Visnu's expansion, Garbhodakashayi Visnu.

All the massive greater Brahmanda universes that originate from Maha Visnu's Body, cover and encase the inner smaller secondary universe within each Brahmanda.

Our small secondary universe is called Bhu-Mandala and has 14 planetary systems according to Srimad Bhagavatam's 5th Canto, and is 4 billion miles (500,000,000 Yojanas) in diameter.

One yojana equals 8 miles.

Sadaputa dasa ACBSP (Richard L. Thompson) earned his Ph.D. in mathematics from Cornell University. He is the author of several books on Vedic Cosmology explain what Bhu-Mandala is-

"In the Bhagavatam, Bhu Mandala—the "earth mandala" (not to be confused with our small earth globe), is a disk 500 million yojanas in diameter.

Our small earth global round sphere is only 24,901 miles in circumference and is 7,917.5 miles in diameter.

Therefore our small earth globe is NOT Bhu-Mandala that is described in Srimad Bhagavatam as 500 million yojanas in diameter (4 billion miles in diameter or 12.6 billion miles in circumference)

As said above, a yojana is a unit of distance that is about 8 miles long, and so the diameter of Bhu Mandala is about 4 billion miles which is the size of our single universe according to Srimad Bhagavatam.

Bhu-Mandala is marked by circular features designated as islands (dvipas that can also mean planets) and oceans.

Srila Prabhupada - "The "planets" are called "dvīpas." Outer space is like an ocean of air. Just as there are islands in the watery ocean, these planets in the ocean of space are called dvīpas, or islands in outer space." (CC Madhya 20.218, Purport)

Bhu-Mandala is far too big to be our small earth globe Sadaputa has explained.

Our secondary single universe with its 14 planetary Systems would therefore fit in the circumference of Bhu-Mandala. (4 billion miles or 500,000,000 Yojanas in diameter)

Sadaputa dasa - "Bhu-Mandala innermost island, Jambudvipa, contains Bharata-varsha which Srila Prabhupada has repeatedly identified as our earth globe. The first covering of our outer universe (Brahmanda) that surrounds and encases our inner single universe, is earth, then water, then fire, then air, then sky, etc." (SB Canto 6 Ch 16 text 37)

In most references in Srimad Bhagavatam it is said the ''earth covering'' is the first layer of our inner universe's surrounding Brahmanda shell although in other places in Bhagavatam it says water as found in Srimad Bhagavatam Canto 3 Chapter 26 text 52.

There are many contradictory comments in Srimad Bhagavatam like this that needs further understanding, such as in many texts it says there is 7 layers covering our inner material universe while in other places it says 8 layers.

The same with Brahma's day/night where it says it is 8 billion 640 million human years long while other places it says 8 billion 600 million human years long.

There are billions of Brahmanda greater universal shells, each with a smaller secondary universe deep inside them that many planetary systems. 

In our Bhu-Mandala universe deep inside our Brahmanda, there are 14 planetary systems.

As said above, all Brahmanda universes originate from the Body of Maha Vishnu as seen in photo below.

Srimad-Bhagavatam Canto 3 Chapter 26 text 52 says water is the first layer.

However, most references say the seven layers of the Brahmanda our secondary universe is deep within, begins with earth.

Srimad Bhagavatam Canto 6 Chapter 16 text 37-

"Every universe is covered by seven layers—earth, water, fire, air, sky, the total energy and false ego—each ten times greater than the previous one. 

There are innumerable universes besides this one, and although they are unlimitedly large, they move about like atoms in You. Therefore You are called unlimited [ananta]."

Purport 

Srila Prabhupada - "This verse describes the coverings of the universe (saptabhir daśa-guṇottarair aṇḍa-kośaḥ)

1 - First is earth,

2 - Second is water,

3 - Third is fire,

4 - Fourth is air,

5 - Fifth is sky,

6 - Sixth is the total material energy known as the mahat-tattva.

7 - Seventh is the false ego.

Beginning with the covering of earth, each covering is ten times greater than the previous one. Thus we can only imagine how great each universe is, and there are many millions of universes." (SB Canto 6 Ch 16 text 37)

Srila Prabhupada - "The mahat-tattva, the total material energy, is called avyakta. Then, when it is agitated by the glance of the Supreme Lord, the three modes of material nature acts. 

And by interaction of these three modes of material nature, the whole cosmic manifestation comes into being." (Lecture on SB 1.2.33 - Vrndavana, Nov 12, 1972)

Srila Prabhupada - "So actually, there are twenty-six different elements which is conducting this material world. Etat kṣetraṁ samāsena sa-vikāram udāhṛtam. Kṣetra means this body. Either you take this body or this material world, they are interaction of these twenty-four elements.

Either you take the gigantic body of this universe or you take the body of this planet or you take this your body, my body, or a cat's body, dog's body, all these bodies, they are formed of these twenty-four elements, sa-vikāram, by action and reaction.

Just like chemically, if you mix one chemical with another chemical, a third element is produced, similarly, originally the reservation of all these elements is called mahat-tattva. It is called pradhāna, upadhāna.

So gradually they manifest, they divide by three guṇas. Three guṇas means in the mahat-tattva, in the total material reservoir, three guṇas, three modes of nature, first of all appear and they act with one another, and then gradually, one after another, the twenty-four elements become manifested. Etat kṣetraṁ samāsena sa-vikāram udāhṛtam." (Lecture on BG 13.6-7, Montreal, Oct 25, 1968)

Srila Prabhupada - "So mahat-tattva, the total material energy, and six, five elements, sixteen, sixteen, ṣoḍaśa-kalam.

What are those?

The five elements, namely, earth, water, fire, air, and sense objects and senses and the spirit soul. 

In this way, sixteen. And from sixteen, it expands to twenty-four. That is the explanation of our Vedic creation. Ṣoḍaśa-kalam." (Lecture on SB 1.3.1 - Vrndavana, Nov 14, 1972)

Srila Prabhupada - "The total energy of material creation is called mahat-tattva or pradhāna. Then, when the mahat-tattva is agitated by the three guṇas, then they become divided into twenty-four elements." (Lecture on SB 3.26.11-14, Bombay, Dec 23, 1974)

Srila Prabhupada - "That mahat-tattva is above this universe, above the sky. Above the sky there are seven layers.p Each layer is ten times more than the other layer. In this way, that is called mahat-tattva. Total material elements, they are stocked there. And then these varieties take place. And above that mahat-tattva, there is spiritual world." (Lecture on SB 3.26.10 - Bombay, Dec 22, 1974)

As confirmed by the Lord Himself in Bhagavad-gītā Chapter 10 text 42-

"But what need is there, Arjuna, for all this detailed knowledge? With a single fragment of Myself I pervade and support this entire universe. The entire material world manifests is only one fourth of the Supreme Lord's energy. Therefore He is called ananta."

Srimad-Bhagavatam Canto 2 Chapter 2 text 28 Purport-

Srila Prabhupada - "The extent of the cosmic phenomenon or our individual universe is calculated to be diametrically four billion miles in size.

Then the outer covering known as the ''Brahmanda shell'' begins.

The first covering is the earth element, calculated to extend forty billion miles, (10 times the size of the inner universe) and the subsequent coverings of the universe are respectively of 

water, 

fire, 

air, 

sky, 

total material energy, 

false ego. 

One after another, each extending ten times further than the previous.

The fearless devotee of the Lord penetrates each one of them and ultimately reaches the absolute atmosphere where everything is of one and the same spiritual identity.

Then the devotee enters one of the Vaikuṇṭha planets, where he assumes exactly the same form as the Lord and engages in the loving transcendental service of the Lord.

That is the highest perfection of devotional life. 

Beyond this there is nothing to be desired or achieved by the perfect yogī." (SB Canto 2 Ch 2 Text 28 Purport)

Srimad Bhagavatam Canto 3 Chapter 29 Text 43-

Srila Prabhupada  - "The first layer is of EARTH, and it is ten times greater in size than the space within the universe;

The second layer is WATER, and that is ten times greater than the earthly layer;

The third covering is FIRE, which is ten times greater than the water covering. In this way each layer is ten times greater than the previous one." (SB Canto 3 Ch 29 Text 43)

Srimad Bhagavatam says there are millions of individual Brahmanda universes coming from Maha Vishnu making up the entire material creation (1/4 of the Spiritual Sky) that also house their own inner universe.

All smaller universes inside their Brahmandas are of different sizes, some, like ours, are only 4 billion miles in diameter.

The "4 billion miles (500,000,000 Yojanas)" diameter of our universe with its 14 planetary systems is one of the smallest.

Srila Prabhupada - "We have to imagine that these universes (Brahmandas), which according to our limited knowledge are expanded unlimitedly, are so great that the gross and subtle ingredients—the five elements of the cosmic manifestation, namely earth, water, fire, air and sky, along with the total material energy and false ego—are NOT only within the universe but cover the universe in seven layers, each layer ten times bigger than the previous one.

In this way, each and every universe is very securely packed, and there are numberless universes. All these universes float within the innumerable pores of the transcendental body of Mahā-Viṣṇu.

It is stated that just as the atoms and particles of dust are floating within the air along with the birds and their number cannot be calculated, so innumerable universes are floating within the pores of the transcendental body of the Lord. For this reason, the Vedas say that God is beyond the grasp of our knowledge." (Krsna Book 87)

We cannot see from one material universe to another. 

This is because our inner material individual universe, deep inside our Brahmanda, is surrounded and encased by that Brahmanda. 

So it is impossible to see the millions of other Brahmandas outside our inner universe (inside a massive Brahmanda) because of those 7 thick material layers that only rare mystic yogis and advanced devotees like Arjuna can see and pass through.

So of course Krsna and Arjuna did not encounter such limitations that we experience with the combination of these elements like earth, water, fire all gross elements as Bhagavad Gita explains.