There is a recent thread on this "Nephilim’ in the back fence board, it has some great information.
Briefly, you are right, fallen angels cannot have sex with human females, BUT they can possess human males and those men had sex with the females, and along with the human ‘seed’, there was also some ‘demonic material’ passed into them, thus, they gave birth to half human, half fallen angel hybrids, who happened to be very big. How this all happened is beyond any of us, we can not even begin to speculate the logistics of this). It was Satans attempt to corrupt the human seed so Jesus could not be born, but God bested him with the virgin birth! (I can imagine satan saying D’Oh) lol
Personally I believe they were around 10-20ft tall, but a particular verse in Enoch says they are something like 350 cubits tall, which would make them over 450ft tall, I dont believe they were THAT big though. Its said they all had double rows of teeth, and some say red hair.
What I find curious about this, is the bible says they existed before AND AFTER the flood, Im not sure why God didnt kill them all off…??
one bible verse gives the dimensions of one of their beds, (it was 9ft long).
You will find this topic is debated though, as some do not believe giants actually existed
Here is the verse…
Enoch chapter 7:12-15
Whose stature was each three hundred cubits. These devoured all which the labor of men produced; until it became impossible to feed them;.