Once while Krsna was ruling Dvaraka, Lord Brahma from our material universe came to see Him.

Our small material universe, created by Garbhodakashayi Visnu, is maintained by our Lord Brahma who builds the planetary systems inside it.

Furthermore, our material universe is deep inside a larger surrounding universe called a Brahmanda that comes from the Body of Maha-Visnu.

There are billions of these massive Brahmanda universes each with a secondary smaller material universe inside of them.

The doorman of Dvaraka immediately informed Krsna of Brahma’s arrival.

When Krsna was informed, He immediately asked the doorman- 

Krsna - "Which Brahma is it? What is his name, what universe is he from?"

The doorman returned and questioned Lord Brahma of which Brahma he was.

The doorman asked - "Which Brahma are you, which universe are you from?"

Lord Brahma was very surprised as he believed he was the only Brahma.

Lord Brahma said - "Please inform Krsna that I am the four-headed Brahma who is the father of the four Kumaras."

The doorman then went and told Krsna what Brahma had said and Krsna then gave Brahma permission to enter. 

The doorman escorted Lord Brahma in, and as soon as Brahma saw Krsna he offered obeisances at the Supreme Lord's Lotus feet.

After being worshiped by Brahma, Krsna also honored Lord Brahma with suitable words. 

Krsna asked - "Why have you come to visit me?"

Brahma replied - "I will later tell You why I have come but first I have some doubts in my mind which I wish You too kindly answer why did you inquire which Brahma had come to see You? What is the purpose of such an inquiry? Is there any other Brahma besides me within the universe?"

Upon hearing this, Krsna smiled and closed His eyes in meditation.

Then immediately an unlimited number of Brahmas appeared instantly to the great suprise of our Brahma.

Furthermore all these Brahmas had a different numbers of heads than our Brahma's four heads. 

Some had ten heads, some twenty, some a hundred, some a thousand, some had ten thousand, some had a hundred thousand, some had ten million and had hundreds of millions. 

No one can count the number of faces they had and Lord Brahma was totally amazed at what he was seeing.

To Brahma's surprise, there also arrived many Lord Sivas with various heads numbering one hundred thousand and also some with ten million!

Many Indras also arrived, and they had hundreds of thousands of eyes all over their bodies.

When our four-headed Brahma from our single material universe situated deep inside our surrounding Brahmanda, saw all these opulences of Krsna, he became very bewildered and considered himself to be very small and insignificant in a great material universal creation that was much bigger than he could ever imagined existed!

As these Brahmas that arrived to see Krsna, offered their respects at Krsna’s lotus feet, and when they did this, their helmets touched His lotus feet.

No one can estimate the inconceivable potency of Krsna the Supreme Personality of Godhead and cause of all causes. 

All the Brahmas who were there,  were resting in the one body of Krsna.

When all their helmets struck together at Krsna’s ’s lotus feet, there was a tumultuous sound. 

It appeared that the helmets themselves were offering prayers unto Krsna’s lotus feet.

With folded hands, all the Brahmas and Sivas began to offer prayers unto Krsna, saying, "O Lord, You have shown me a great favour. I have been able to see Your lotus feet."

All the Brahmas then said - "It is my great fortune, Lord Krsna, that You have called me, thinking of me as Your servant. Now let me know what Your order is so that I may carry it on all my heads."

Krsna replied - "Since I wanted to see all of you together, I have called all of you here, all of you should be happy. Is there any fear from the demons?"

They replied - "By Your mercy, we are victorious everywhere, whatever burden was upon us You have taken away by descending on my planet in my universe."

All the Brahmas then believed that Krsna was always staying in their jurisdiction.

None of the Brahmas could see the other Brahmas except for our Brahma who Krsna allowed to be seen so he could understand he is not the only Lord Brahma.

Thus the opulence of Krsna's abode Dvaraka was perceived by each and every one of the Lord Brahmas. 

As already explained, although they were all assembled together, none of the Lord Brahma could see any other Brahma.

Krsna then bade farewell to all the Brahmas, and after offering their obeisances, they all returned to their respective material universes that exists deep inside their respected larger Brahmanda universes.

As said above, after observing all these opulences, our four-headed Brahma from our universe was astonished. He again came before the lotus feet of Krsna and offered Him obeisances.

Brahma said - "Whatever I formerly decided about my knowledge, I have just now had personally verified -

jananta eva janantu

kim bahuktya na me prabho

manaso vapuso vaco

vaibhavam tava gocarah

"There are people who say, "I know everything about Krsna." Let them think in that way. As far as I am concerned, I do not wish to speak very much about this matter. O my Lord, let me say this much. As far as your opulence’s are concerned, they are all beyond the reach of my mind, body and words." (Srimad-Bhagavatam Canto 10 Chapter 14 text 38)

Krsna says - "Your particular universe extends four billion miles in diameter; therefore it is the smallest of all the material universes. Consequently you have only four heads."

Some of the secondary universes (inside the Brahmanda) are 4 billion miles in diameter, some are 8 billion, some 80 billion miles etc etc. Thus they are almost unlimited in area.

And the size of the outer Brahmanda universes are different too, some are billion of times bigger than our Brahmanda.

And as already explained, the size of each secondary material universe inside their larger Brahmanda, is determined by the number of heads each Brahma has. 

Maha-Visnu maintains innumerable material universes (Brahmandas) originating from His bodily pores and breathing.

Krsna explains - "No one can measure the length and breadth of one fourth of My energy. And who can measure the three fourths that is eternally manifest in the spiritual world?"

Beyond the River Viraja is a spiritual nature, which is indestructible, eternal, inexhaustible and unlimited. 

The Supreme Abode consists of three fourths of the Lord’s opulences and is known as paravyoma, the Spiritual Sky."(Adapted from Sri Caitanya Caritamrta Madhyalila chapter 21)**