What is the size of the observable universe? How do scientists know how big the invisible portion is if they can't see it?
Good question! although, what scientists see is just a hypothesis...
Scientists don't know the size of the universe. They just imagine it to be infinite because of the number of stars and planets it can accommodate.
Every time an astronomer looks at the space through a telescope, he or she discovers a new galaxy, probably every hour! What you are saying maybe right because scientists are judging the size of the unseen parts of the universe, to be infinite.
Any certain claim made about the space is mostly a hypothesis!