Charles Ellicott Commentary Genesis 6:15

"And this is how thou shalt make it: the length of the ark three hundred cubits, the breadth of it fifty cubits, and the height of it thirty cubits." — Genesis 6:15 (ASV)

Cubits. —The cubit is the length of the arm from the elbow to the tip of the middle finger. Additionally, as it was regarded as one-fourth of a man’s height, we can safely compute it at eighteen inches, except where the sacred or longer cubit is expressly mentioned. Thus, the ark was 450 feet long, 75 feet wide, and 45 feet in depth. The Great Eastern is much larger, being 680 feet in length. However simple her construction, there would have been great difficulty in building so large a vessel, from the danger of her breaking her back, especially in the tempestuous weather that followed.