When you talk about the word "big" then you refer to size and measurement. Well, to be honest, it's hard to imagine how big is the universe. What scientist can estimate so far is not even the end of the universe yet. One way how the observable universe can be measured and estimated is by using scaling diagram.
In this lesson you will be exposed to some resources which can help you build a better sense of how big is our universe.
Look at the picture below
This video is made in 1977 but it illustrates clearly about how big is our universe with zoom out and zoom in techniques. Watch carefully and record the numbers shown the size/ scale of each part of the universe.
When you finished exploring the resources, then you can explain how big our universe is by writing an essay. If you prefer drawing then you can draw/ illustrate or create a visual diagram to show the estimation of how big the universe is.
Have fun learning!