I think you are confusing two things.
First, the size of the universe. We don't know how large it is. It could theoretically go on forever, or it could have a certain determinable size, or whatnot. However, we can not presently see all of it, so we can't say for sure. An infinitely large universe poses no direct problem.
Second, the time the universe has been around. It is impossible for the universe to have been around forever before now. If the universe is 13 billion years old, you see all the stars 13 billion light years or closer. However, if the universe would be infinitely old (eternal) you would see all stars from infinitely billion light years or closer. Which we don't.