How big is the universe? is it infinite? if not, how does it end?
80 - 90 billion light years, no, unknown
Astronomers can only guess at the full extent of the universe. Their latest guestimates puts it at 80 - 90 billion light years large. The limiting factor to this guess is how much of the universe we are actually seeing with the instruments available to us.
It is believed the universe is finite otherwise you could not give it a size as above.
No one knows how it will end. One theory has it expanding until the very atoms we are made up of are ripped apart. This is the prevailing belief.