God orders an ark. And, since there’s only one family listening to God at the moment, Noah’s family is to build it.
An ark is a boat. A big boat. A HUGE boat! It was around 500 feet long, over eighty feet wide, and fifty feet tall. Humans would not build another boat its size for over 3500 years. No wonder it took 120 years to build.
This boat would hold Noah’s family, plus either two or seven of every animal kind on the face of the planet. Note that I did not say “species”. A kind is a much broader description. There are dozens of species of cats in the world. There are lions, tigers, pumas, jaguars, and house cats, to name a few. But, there would only be one or two cat kinds. There was probably one kind of dogs, and maybe 50 kinds of dinosaurs.
All in all, there were probably about 8,000 kinds of land animals and birds on the ark. Since they came in twos and sevens, that was probably between 20,000 and 30,000 animals.
The average size of those animals was probably in the neighborhood of a sheep or larger dog. So, they would easily fit into the ark.
But, that begs the question, why was the ark so big? While it’s not mentioned, there also needed to be food for all of those animals as well. And, for the humans. And, they would be on the boat longer than a year, so that’s a lot of food. And, don’t forget fresh water for drinking. All in all, this boat was probably sized exactly correctly, since it was God who designed it, and He knew what was needed.