How big is the Universe?
Nobody knows for certain how big the Universe is because we cannot see to the edge of it. We don't even know if it has an edge. We can only see out to a distance of about 14 billion light years from Earth. This means that the size of the Universe that we can see is about 28 billion light years in diameter (across). However this distance is determined from things we can measure and infer their distance near at the time of the Big Bang. Taking into account how the universe has been expanding and how the rate of expansion may have changed - a matter of ongoing research among astronomers - then the size of the observable universe is actually 92 billion light years!
The size, shape, expansion rate and even what is inside the Universe comprises a large portion of ongoing modern astronomy research. To get started on exploring some of these potentially mind-boggling concepts, explore the NASA Astrophysics Science website.