Why is it important to know the size of the universe?
Astro-physicists right now can only guess at the size and shape of the universe. There is no consensus right now as to either.
Some think the universe is pancake shaped while other think it is spherical shaped like a soccer ball.
Problematic to them is "seeing" the furthest galaxies. Right now they have identified galaxies at about 45 billion light years away. Why this is a problem is that the universe is known to be about 13.7 billion years old. That would suggest we can only see objects at a distance of 13.7 billion light years away.
Scientists know that the universe is expanding at an ever increase rate but what exactly that rate is, they do not know. They do theorize, that within the first few seconds of existence after the big bang, the universe expanded to half the size it is right now. And it has been expanding ever since which means that those galaxies which are 45 billion light years away are moving away from us at a very fast speed.
Scientist also refer to those outer reaches as being part of the "visible universe." They suggest there may well be, and probably are, galaxies further out than 45 billion light years that we have not been able to detect yet for whatever reason.
There are far too many holes in our knowledge of the universe to worry about knowing how big it is.