In Genesis 6:15–16, God tells Noah very specifically how to build the ark:
(New International Version)
Now, suppose you're Noah. The Flood is coming. God has told you to make the ark three hundred cubits long. Is God just offering a helpful estimate here, about how big the ark will need to be? Or is this one of those situations where you have to measure exactly, because God said “three hundred cubits” and not “roughly three hundred cubits”?
The Flood is coming, Noah is in a big hurry, but has had to measure that three hundred cubits exactly. Because if he made it a handsbreadth too long or too short, he might have turned out to be the guy cited in the second edition of the Bible as having caused the destruction of the world by being insufficiently attentive to the command of God.