The first purusa-avatara is Maha-Visnu, and that Maha-Visnu is the plenary portion of the plenary portion of the Supreme Personality of Godhead, Krsna

Expressions researched:
"The first puruṣa-avatāra is Mahā-Viṣṇu, and that Mahā-Viṣṇu is the plenary portion of the plenary portion of the Supreme Personality of Godhead, Kṛṣṇa"

Srimad-Bhagavatam

SB Canto 5

Every living being is controlled by the supreme living being, Paramātmā, who resides within everyone's heart. He is the puruṣa, the puruṣa-avatāra, who creates this material world. The first puruṣa-avatāra is Mahā-Viṣṇu, and that Mahā-Viṣṇu is the plenary portion of the plenary portion of the Supreme Personality of Godhead, Kṛṣṇa.

SB 5.11.13-14, Translation and Purport

There are two kinds of kṣetrajña—the living entity, as explained above, and the Supreme Personality of Godhead, who is explained as follows. He is the all-pervading cause of creation. He is full in Himself and is not dependent on others. He is perceived by hearing and direct perception. He is self-effulgent and does not experience birth, death, old age or disease. He is the controller of all the demigods, beginning with Lord Brahmā. He is called Nārāyaṇa, and He is the shelter of living entities after the annihilation of this material world. He is full of all opulences, and He is the resting place of everything material. He is therefore known as Vāsudeva, the Supreme Personality of Godhead. By His own potency, He is present within the hearts of all living entities, just as the air or vital force is within the bodies of all beings, moving and nonmoving. In this way He controls the body. In His partial feature, the Supreme Personality of Godhead enters all bodies and controls them.

This is confirmed in Bhagavad-gītā (BG 15.15). Sarvasya cāhaṁ hṛdi sanniviṣṭo mattaḥ smṛtir jñānam apohanaṁ ca. Every living being is controlled by the supreme living being, Paramātmā, who resides within everyone's heart. He is the puruṣa, the puruṣa-avatāra, who creates this material world. The first puruṣa-avatāra is Mahā-Viṣṇu, and that Mahā-Viṣṇu is the plenary portion of the plenary portion of the Supreme Personality of Godhead, Kṛṣṇa. Kṛṣṇa's first expansion is Baladeva, and His next expansions are Vāsudeva, Saṅkarṣaṇa, Aniruddha and Pradyumna. Vāsudeva is the original cause of the brahmajyoti, and the brahmajyoti is the expansion of the rays of the body of Vāsudeva.

yasya prabhā prabhavato jagad-aṇḍa-koṭi-

koṭiṣv aśeṣa-vasudhādi-vibhūti-bhinnam

tad brahma niṣkalam anantam aśeṣa-bhūtaṁ

govindam ādi-puruṣaṁ tam ahaṁ bhajāmi

"I worship Govinda, the primeval Lord, who is endowed with great power. The glowing effulgence of His transcendental form is the impersonal Brahman, which is absolute, complete and unlimited and which displays the varieties of countless planets, with their different opulences, in millions and millions of universes." (BS 5.40) The Supreme Personality of Godhead is thus described in Bhagavad-gītā:

mayā tatam idaṁ sarvaṁ

jagad avyakta-mūrtinā

mat-sthāni sarva-bhūtāni

na cāhaṁ teṣv avasthitaḥ

"By Me, in My unmanifested form, this entire universe is pervaded. All beings are in Me, but I am not in them." (BG 9.4)

This is the position of the plenary expansions of Kṛṣṇa as the all-pervading Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha.