Maha-Visnu is surrounded by millions of massive almost spherical material universes called Brahmandas.

Deep inside each Brahmanda universe there is a "secondary smaller universe" where Garbhodakashayi Visnu, Lord Brahma and many planetary systems exist.

From Maha-Visnu's pores and breathing, millions of massive almost spherical universes originate called Brahmandas.

As said above, deep inside each one of them is another smaller universe where Garbhodakashayi Visnu and Lord Brahma reside.

Also within each material universe, deep inside their surrounding Brahmanda, exists many planetary systems built by Lord Brahma and his demigod associates.

The size of each single material universe depends on how many heads Lord Brahma has.

Our single universe is called Bhu-Mandala and has 14 planetary systems, and our Brahma has only 4 heads.

Other universes, deep inside of their Brahmanda, Brahma ha many heads, some with 10, 20, 500, 1000, 100,000, a million, or even 10 million heads.

Such knowledge is inconceivable for us to properly understand.

This means some inner material universes and their outer surrounding Brahmanda shell, are millions of times bigger than our Bhu-Mandala universe and its surrounding Brahmanda shell covering it.

All this knowledge is found in Srimad Bhagavatam 5th Canto translated by His Divine Grace A.C. Bhaktivedanta Swami Prabhupada.