how do the sientists mesure the universe if they dont know the center of it or the end?

well, its all theories. most of the theories are based on the big bang theory, theory of relativity, expansion thoery and so on. Based on all these phenomenons we come with the size of the universe.

edit: and if those theories are true, then the given size of the universe must also be true. and yeah there is no exact number as it is impossible to for now.

Astronomers estimate that the Big Bang occurred between 10 and 20 billion years ago. They estimate the age of the Universe in two ways: (a) by looking for the oldest stars; and (b) by measuring the rate of expansion of the Universe and extrapolating back to the Big Bang.

There is reasonable correlation between the two methods.

There are two main theories, the "soccer ball" and the "expanding flat" one. They are based on the analysis of light and being able to estimate the age of the oldest known light. The speed of light has been measured, so they extrapolate based on the time it took for that light to travel and where it traveled from. Unfortunately, the expanding universe theory has no limit on the speed of expansion. It could be getting bigger faster than the speed of light and the light would not be able to catch up and reach us to be measured making it vastly larger than thought. The soccerball theory could also be true if it is assumed the size is larger than the current estimate of the expanding universe-156 billion light years wide.

The soccer ball one is as bewildering as pondering the infinite universe, if the universe is closed, then what is beyond it? lol