There seems to be a resurgence of the teaching that evil spirits are disembodied giants, the offsprings of the sons of God and the daughters of men.
There typically isn't a scriptural reason given for this doctrine, but generally an appeal to an extra-biblical source, i.e. the book of Enoch.
That is then usually followed up by how many people, two thousand years ago, believed the book should be considered in the canon of the scripture.
Then there is usually some name dropping, of early church writers or rabbis, who treated the book of Enoch as scripture.
All that is often done to justify that the only source that they have for their doctrine is the book of Enoch, and not from the scriptures.
So, do the scriptures mention anything identifying the spirits?

Hebrews 1:7 And of the angels he saith, Who maketh his angels spirits, and his ministers a flame of fire.
 13 But to which of the angels said he at any time, Sit on my right hand, until I make thine enemies thy footstool?
 14 Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?

The angels are spirits, as seen in line 7 & 13, above.
Not all the angels are good:

Psalm 78:49 He cast upon them the fierceness of his anger, wrath, and indignation, and trouble, by sending evil angels among them.

There are such things as evil angels that are sent among people, see above.
So if the angels are spirits, and there are evil angels, then what about evil spirits?

Acts 19:12 So that from his body were brought unto the sick handkerchiefs or aprons, and the diseases departed from them, and the evil spirits went out of them.

Is it not easier to see that those evil spirits are actually evil angels?
So, without going outside the scriptures for doctrine, how does one then teach that evil spirits are disembodied giants, rather than evil angels?
Evil angels seem to follow the devil:

Matthew 25:41 Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:

Revelation 12:7 And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
 8 And prevailed not; neither was their place found any more in heaven.
 9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
 
The book of Enoch ought not be the source for doctrine, that is not found in the scriptures.
If things in that book back up and corroborate doctrines that already are found in the scriptures, that's different.
However, it ought not be the source for doctrine.