Genesis 6, On Nephilim
The Bible doesn't sufficiently answer all of our questions here. As is the case with other passages like this, we are better off focusing on what the Bible does say rather than speculating about what it doesn't.
The Holy Spirit's concern is that you find the profit of the teaching, reproof, correction, and training for righteousness that the passage plainly provides so that God's people may be "equipped for every good work (2 Tim. 3:16)." So, you may never know what size shoe a Nephilim wears, but you would do well to pay attention to what the passage plainly says.
What Genesis 6 plainly says is that there were mighty men on earth at the time. They were legendary "men of old." They were men of renown. Everyone knew who they were.
But they were not more mighty than God.
Now, no one knows who they were.
As mighty and well-known as they were, these "men of old" didn't survive the flood. If the wrath of God got them, it will get you too. Pay attention!
Throughout human history, people have been prone to create a new class of legendary figures. These people tend to be bigger than the rest. Perhaps they are smarter than the rest. Either way, they tend to have more money than the rest.
But they sin like the rest.
They die like the rest.
They will give an account to God like the rest.
We see them as heroic, untouchable, and bigger than life. Genesis 6 sees them as failures who could not survive the flood.
They may be bigger than you, but they are wicked like you.
As such, God's judgment is just. When it comes to God's wrath, the legends were not sufficient swimmers to save themselves. So where does that leave me and you?
Death is the great equalizer. No one is untouchable. We all need to be saved. God has revealed both His wrath against wickedness and His way of salvation. He is just both in His wrath on wickedness and in His offer of salvation to sinners (Rom. 3:26).
Big sinners or little sinners, Nephilim and sons of God; it all ends the same for sinners. But as God provided Noah a way of salvation in the ark, so Christ is the way of salvation for us. Repent of sin. Turn to Jesus by faith. Today is the day of salvation.