The existence of the giants will always be, like many other things, a component of the enigmatic human culture, because it is part of the mythology, and as such, it does not make sense to question it scientifically, since we would be mixing knowledge of different statuses.
If there were some humans of uncommon stature, as manifested by fossils found, it seems to me irrelevant to assess the issue at hand.
I have read the Biblical Old Testament not as a specialist (of course in versions translated into my mother tongue: Spanish) and I don't remember mentioning a giant before Goliath; the closest would be Goliath. But, as someone says, the figure of the "giant" would not be a metaphor for excessive power. I remember that in Mayan mythology, a community kills a character called Zipacná for this boasting of his enormous strength (the equivalent of Greek excess -hybris-).
There is in Sumerian literature an interesting precedent in the Epopee of Gilgamesh.
Thank you for your interesting post, @ladyrebecca.