And this is the fashion which thou shalt make it of: The length of the ark shall be three hundred cubits, the breadth of it fifty cubits, and the height of it thirty cubits.

Jamieson-Fausset-Brown Bible Commentary

15. And this is the fashion-According to the description, the ark was not a ship, but an immense house in form and structure like the houses in the East, designed not to sail, but only to float. Assuming the cubit to be 21.888 inches, the ark would be five hundred forty-seven feet long, ninety-one feet two inches wide, and forty-seven feet two inches high.

Matthew Henry's Concise Commentary

6:12-21 God told Noah his purpose to destroy the wicked world by water. The secret of the Lord is with them that fear him, Ps 25:14. It is with all believers, enabling them to understand and apply the declarations and warnings of the written word. God chose to do it by a flood of waters, which should drown the world. As he chooses the rod with which he corrects his children, so he chooses the sword with which he cuts off his enemies. God established his covenant with Noah. This is the first place in the Bible where the word 'covenant' is found; it seems to mean, 1. The covenant of providence; that the course of nature shall be continued to the end of time. 2. The covenant of grace; that God would be a God to Noah, and that out of his seed God would take to himself a people. God directed Noah to make an ark. This ark was like the hulk of a ship, fitted to float upon the waters. It was very large, half the size of St. Paul's cathedral, and would hold more than eighteen of the largest ships now used. God could have secured Noah without putting him to any care, or pains, or trouble; but employed him in making that which was to be the means to preserve him, for the trial of his faith and obedience. Both the providence of God, and the grace of God, own and crown the obedient and diligent. God gave Noah particular orders how to make the ark, which could not therefore but be well fitted for the purpose. God promised Noah that he and his family should be kept alive in the ark. What we do in obedience to God, we and our families are likely to have the benefit of. The piety of parents gets their children good in this life, and furthers them in the way to eternal life, if they improve it.