Question

what is Universe?what it's size?

Answer 1

all existing matter and space considered as a whole; the cosmos. The universe is believed to be at least 10 billion light years in diameter and contains a vast number of galaxies; it has been expanding since its creation in the Big Bang about 13 billion years ago.
The observable Universe is, of course, much larger. According to current thinking it is about 93 billion light years in diameter.

Answer 2

The proper distance—the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 91 billion light-years (28×109 pc).