Answer:
And this is how you shall make it: The length of the ark shall be three hundred cubits, its width fifty cubits, and its height thirty cubits (GENESIS 6:15).
Abby, your question is often asked by people a lot older than six! First of all, God told Noah how big to make the ark (see our verse). You might ask, what is a cubit? I believe the cubit that was used was about 20 inches long . . . and that would make the ark as long as one and a half football fields—about 510 feet (155 meters)—as tall as a 4-story building—about 45 feet (14 meters)—and about 85 feet (23 meters) wide! It was really big! Now another thing is that although there are hundreds of names of dinosaurs, there were probably only about 50 actual kinds. So, there may have only been a total of 100 dinosaurs on the ark. That still seems like a lot, doesn’t it?
But, did you know that most dinosaurs were actually quite small? In fact, the average size of a dinosaur is the size of a large sheep or bison. (Some were as small as chickens!) And for the few dinosaurs that grew large, it would make sense that God would send smaller young adults. You know, after looking at all of these facts, I think there was plenty of room on the ark for the dinosaurs and all the other animals God sent to survive the Flood.
Source: https://answersingenesis.org/kids/dinosaurs/how-did-all-the-dinosaurs-fit-on-the-ark/