Genesis 6 first gives Noah’s Ark dimensions in the sixth chapter of Genesis.
Genesis 6:15 specifies 300 cubits long, 50 cubits wide, and 30 cubits high. The instructions appear before the Flood begins, within the account of human violence, divine judgment, and Noah’s favor with God.
The next verse describes the Ark’s opening and three decks. Genesis 7 then narrates Noah’s entry, the animals’ arrival, and the onset of the Flood. Genesis 8 covers the recession of the waters, while Genesis 9 presents the covenant after Noah leaves the Ark.
Readers sometimes associate the measurements with Genesis 7 or 8 because those chapters describe the Flood itself. The dimensions, however, are introduced earlier, in Genesis 6, as part of the construction instructions.