How large can the universe ultimately become?
No one knows.
There are a number of differing views on the future of the universe. The most popular is that of the ever expanding universe. But even then there are two differing opinions. One has the last star blinking out and what remains continues to expand forever. The other has everything being ripped apart right down to its most elemental level, quarks. Regardless, with our present knowledge it can only be theory.