How is the size of the universe calculated?

We can only speculate as to whether there is finite or infinite space behind the horizon; if it is infinite, then your answer is most likely that the Universe is infinite, but we can never know for sure. All we currently know 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.

There are a number of ways to calculate the size of the universe, but the most popular one is to measure the observable universe, which is approximately 93 billion light-years in diameter. This measurement is based on observations of the cosmic microwave background radiation and the universe's rate of expansion. Other methods that can be used to estimate the size of the universe include calculating the universe's density of matter and dark energy. It's important to remember that these calculations are based on assumptions about the structure and evolution of the universe and are subject to revision as our knowledge of the cosmos advances.