“This is how you are to make it: the length of the ark 300 cubits, its breadth 50 cubits, and its height 30 cubits.” Gen 6:15.
A cubit was a unit of measurement based on the length between the elbow and fingertips. The cubits varied in length by a few inches according to different civilizations. For example, the Hebrew short cubit was 17.5 inches, while the Egyptian long cubit was 20.6 inches. To learn more about the cubit or see how big Noah’s Ark was, see the following article “What did Noah’s Ark look like?“