What is the basis for gigantic stature of early humans?

Gen 6:4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare [children] to them, the same [became] mighty men which [were] of old, men of renown.

Num 13:33 And there we saw the giants, the sons of Anak, [which come] of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.

Deu 2:10 The Emims dwelt therein in times past, a people great, and many, and tall, as the Anakims;

Deu 2:11 Which also were accounted giants, as the Anakims; but the Moabites call them Emims.

Long before the Bible was written, there were depictions of people carved into the stone of temples and tombs in Egypt. Those pictures carved into the wall had no perspective. Instead, the being depicted were shown at a size proportional to their importance --- even if they were directly interacting with much smaller beings of lesser importance.

Genesis 6:1-8 forms the introduction to the story of the great Flood. It is in two sections, the first of which had no original connection with the Flood narrative. However, the author uses both sections to explain why God sent the deluge.

The first section Genesis 6:1-4 gives a brief summary of an ancient myth that relates how divine beings mated with human women and produced offspring.

In Hebrew thought, the human and the divine ought not to be confused, so these children violate the whole structure of the world. As punishment, God frustrates any attempt on humanity's part to attain divinity and immortality by declaring "my spirit shall not abide in mortals forever", and restricting their lifespan to 120 years.

The children are described as "mighty men of old", who appear in a later chapter as the embodiment of absolute power in the ancient world. But they are also described as "Nephilim", which probably means "fallen ones". This is one source of the myth of the fallen angels that plays a prominent part in such books as Enoch and Jubilees, works that stand outside the Bible and Apocrypha. Jewish thought came to see these angels, rather than the sin of Adam, as the source of sin in the world.

The second section (Genesis 6:3-8) presents a picture of universal wickedness and evil that is the climax of humanity's increasing degeneracy.

There are many accounts of giants in earth's history. Regarding what you're asking, you can find the references in Genesis. But there is speculation that giants built pyramids and many other structures In fact, in the ancient Egyptian and Sumerian artwork giants have been depicted as dwarfing lions (making them look like house cats in comparison), and the blocks of the pyramids (carrying them on their shoulders)

"There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown."- the Word of God. Also, https://www.youtube.com/watch?v=jkNm3jeCPaA and so on.