I'm listening to the book of Enoch right now and some of these jokers were 45 or 450 or 4500 feet tall depending on what translation you are reading. The Watchers had knowledge and could manipulate things so I'd guess they could "shrink" themselves down if needed or their half-spring would grow rapidly after birth but not kill their mothers during pregnancy. One thing of interest is they only took 1 wife from the land dwellers.
