Noah first receives the Ark’s dimensions in Genesis 6. In Genesis 6:14–16, God instructs Noah to build the Ark, specifies gopher wood and pitch, and gives its length, width, height, opening, door, and three decks.
The chapter introduces the reason for the construction: Genesis describes widespread violence and corruption before announcing the coming Flood. Noah is then told to preserve himself, his family, and representatives of living creatures.
Genesis 7 moves from preparation to the Flood itself, while Genesis 8 covers the waters’ retreat and the Ark’s eventual departure. Genesis 9 follows with the covenant symbolized by the rainbow.
Readers sometimes associate the measurements with Genesis 7 because that chapter begins the Flood narrative. The actual construction instructions and dimensions appear earlier, in chapter 6.