Śrīla Sanātana Gosvāmī quotes the following verses from Śrī Harivaṁśa (Viṣṇu-parva 11.18-22), which describe the banyan tree:

dadarśa vipulodagra-
 śākhinaṁ śākhināṁ varam
sthitaṁ dharaṇyāṁ meghābhaṁ
 nibiḍaṁ dala-sañcayaiḥ

gaganārdhocchritākāraṁ
 parvatābhoga-dhāriṇam
nīla-citrāṅga-varṇaiś ca
 sevitaṁ bahubhiḥ khagaiḥ

phalaiḥ pravālaiś ca ghanaiḥ
 sendracāpa-ghanopamam
bhavanākāra-viṭapaṁ
 latā-puṣpa-sumaṇḍitam

viśāla-mūlāvanataṁ
 pāvanāmbhoda-dhāriṇam
ādhipatyam ivānyeṣāṁ
 tasya deśasya śākhinām

kurvāṇaṁ śubha-karmāṇaṁ
 nirāvarṣam anātapam
nyagrodhaṁ parvatāgrābhaṁ
 bhāṇḍīraṁ nāma nāmataḥ

“They saw that best of all trees, which had many long branches. With its dense covering of leaves, it resembled a cloud sitting on the earth. Indeed, its form was so large that it appeared like a mountain covering half the sky. Many birds with charming blue wings frequented that great tree, whose dense fruits and leaves made it seem like a cloud accompanied by a rainbow or like a house decorated with creepers and flowers. It spread its broad roots downward and carried upon itself the sanctified clouds. That banyan tree was like the lordly master of all other trees in that vicinity, as it performed the all-auspicious functions of warding off the rain and the heat of the sun. Such was the appearance of that nyagrodha tree known as Bhāṇḍīra, which seemed just like the peak of a great mountain.”