How big is the whole universe?
We often say: the more you know, the less you know. The same is true of our understanding of the universe.
In ancient Greece, people believed that the earth was the center of the universe, all the sun, moon and stars revolve around the earth, so ancient Greek sages put forward the "geocentric theory". The early geocentric model still had many problems, but with Ptolemy, geocentric theory gradually matured and became the mainstream theory of the time.