When you ask about a "theory" it is important that you not confuse it with a "speculation". The following is a very good definition for "theory".
From https://en.wikipedia.org/wiki/Theory In modern science, the term "theory" refers to scientific theories, a well-confirmed type of explanation of nature, made in a way consistent with scientific method, and fulfilling the criteria required by modern science. Such theories are described in such a way that scientific tests should be able to provide empirical support for it, or empirical contradiction ("falsify") of it.
The following is an acceptable definition of "speculation".
From https://en.wikipedia.org/wiki/Speculation_(disambiguation) Speculation is the process of thinking about possibilities, or a particular conclusion arrived at from such thought.
There was a time (about a century ago) when the theory called General Relativity was born with (at the time) some ideas about how some of the possibilities might be supported by observation. One of these ideas (theories) was (at the time) a finite universe that expanded and reached a maximum size and then contracted. At the present time, there is now a lot of observed data that tells us that this possibility just cannot happen, and no ideas about how new observations will likely occur to support this idea.
On the other hand, a principle of science is that nothing is ever known with absolute certainty (except maybe mathematics). Therefore one might speculate that some time in the future some idea for a theory will come along with a conceptual possibility of observing evidence that will support the idea of a universe reaching a maximum size and then contracting. I would guess that most professional cosmologists would not expect this possibility to have any significant chance of ever happening.