How is the size of the universe calculated?
Depends on what you call our Universe.
All we can now about is our 13 billion lightyears neighborhood, since the Universe is 13 billion years old, and no information can travel faster, than the speed of light. This distance is called the horizon. We can only guess if there is finite or infinite space behind the horizon. If it's infinite, than most probably your answer is: the Universe is infinite. But this is something we can never know.