Inside each Brahmanda is a secondary smaller Universe where Garbhodakashayi Vishnu and Lord Brahma reside.

Here we see in this painting below, millions of Brahmanda single almost spherical massive egg shaped Universes, all coming from the Body of Maha Vishnu's (also known as Karanadaksayi Vishnu) Body.

Deep inside each Brahmanda is a secondary smaller Universe where Garbhodakashayi Vishnu and Lord Brahma reside.

Garbhodakashayi Vishnu and Lord Brahma's smaller Universe is totally surrounded by there particular Brahmanda, like a seed is surrounded by the outer shell layers of a Pine cone.

Lord Brahma's inner smaller Universe is encased and surrounded by the outer Brahmanda Universe that is made up of 7 massive material layers.

These 7 material layers of the Brahmanda are collectively 44 quadrillion 444 Trillion, 444 billion miles in diameter. 

However if the measurement is taken from the outer edge of the smaller inner universe where Garbhodakashayi Vishnu and Brahma reside, then the full diameter of the Brahmanda is 88 quadrillan, 888 trillion, 888 billion.

Some inner universes, that are all very deep within their Brahmanda, are very small in comparison to the outer Brahmanda surrounding it, some are just a mere 4 billion miles in diameter according to the 5th Canto of Srimad Bhagavatam.