Unlike many whimsical drawings that depict the Ark as some kind of overgrown houseboat (with giraffes sticking out the top, for example), the Ark as described in the Bible was a big vessel. Not until relatively recent times (in the late 1800s) was a ship built—using steel—that far exceeded the capacity of Noah’s Ark.
The dimensions of the Ark are convincing for two reasons: the proportions are like that of a modern cargo ship, and it is about as large as a wooden ship can be built. The cubit gives us a good indication of size. (The cubit was defined as the length of the forearm from elbow to fingertip. Ancient cubits vary anywhere from 17.5 inches to 22 inches, with the longer sizes dominating the major ancient construc- tions. Even a conservative 18-inch cubit describes a sizable vessel.)
We know the Ark must have been at least 450 feet long, 75 feet wide, and 45 feet high. Using a longer cubit, it would have been over 500 feet in length. In the Western world, wooden sailing ships never got much longer than about 330 feet, yet much earlier the ancient Greeks were building vessels at least this size. China built huge wooden ship s in the 1400s that may have been as large as the Ark. The biblical Ark is one of the largest wooden ships of all time, a mid-sized cargo ship even by today’s standards.
The Ark had three decks (Gen. 6:16), so this gives you an idea of its overall size. With a capacity of 1.5 million cubic feet, it would have had sufficient room for all the people, animals, and supplies.
By Ken Ham and Tim Lovett, Answers in Genesis
Ray Comfort's Daily Evidence 
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