Damon in pages 155-57 has an exhaustive display of
Blake Giants. Here are few things I found especially
noteworthy. Blake (like most ordinary Christians
today) did not think well of the O.T, theologically
speaking. However he found it rich in ideas and
meanings, and he had no hesitation about drawing on
them in his own creations. There are two kinds of
giants in the Bible:
1. Antediluvian (pre Flood)
 And it came to pass, when men began to multiply on
the face of the earth, and daughters were born unto
them.  That the sons of God saw the daughters of
men that they were fair; and they took them wives of
all which they chose.
 And the LORD said, My spirit shall not always
strive with man, for that he also is flesh: yet his
days shall be an hundred and twenty years.  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 Book of Enoch, which Blake had undoubtedly read,
has material about these giants. (If you want to
research Enoch, try chapter 7; good luck).
One of Blake's first mentions of the Giants comes in
Plate 16 (lines 2-8) of the Marriage of Heaven and
"02 The Giants who formed this world into its
03 sensual existence and now seem to live in it
04 in chains, are in truth. the causes of its life
05 & the sources of all activity, but the chains
06 are, the cunning of weak and tame minds. which
07 have power to resist energy, according to the pro-
08 -verb, the weak in courage is strong in cunning."
And in Plate 17, Lines 8-10 we read that "Messiah or
Satan or Tempter was formerly thought to be one of the
Antediluvians who are our Energies".
"The giants symbolize the great primeval powers within
us, although mostly hidden within our bodies..., also
the great thinkers, those individuals who have overcome
these [fleshy] limitations and realized their powers."
(Damon 155) (No doubt Blake at times thought of himself
in that category.)
Blake saw these antediluvians as people rather than
states: they could not be annihililated, but he put
them at the bottom of the Sea of Time and Space.
2. The Postdeluvian Giants included the giant Albion,
"sunk in the deadly sleep of materialism" (Damon 156)
I've copied 4 lines from Jerusalem plate 38: "hear the
Giants of Albion cry at night
We smell the blood of the English! we delight in their
blood on our Altars! The living & the dead shall be
ground in our rumbling Mills 51 For bread of the Sons
Shades of Jack and the Beanstalk!
And from Jerusalem Plate 49:
07 Come & mourn over Albion the White Cliff of the Atlantic
08 The Mountain of Giants; all the Giants of Albion are become
09 Weak! witherd! darkend! & Jerusalem is cast forth from Albion.
10 They deny that they ever knew Jerusalem, or ever dwelt in Shiloh
The subject deserves more study. Damon made a good beginning, but there's still a lot of work to do. However you may readily perceive that Blake took the Giant symbol and used it to portray much of the fallenness which describes our materialistic world.