Closed--you can come back to where you started by going all the way in one direction. Very small--the expansion of the universe, if inevitable, will not be able to prohibit this kind of round-about travel
Is that even possible?
if yes, what will the twin paradox look like?
if no, why not?
While the question about the twin paradox in a closed universe is a duplicate, I think it's worth a few comments to clarify what is meant by a closed universe.
There are two different meanings for the word closed when used in connection with the universe. Observations suggests the universe is at least approximately described by the FLRW metric, and this geometry can be flat, open or closed depending on the density of matter. However in a closed FLRW universe you cannot travel in a straight line and return to your starting point. The description closed means that everything in the universe will ultimately meet in a future singularity (though note that this future singularity isn't a point any more than the Big Bang was).
The other meaning for the work closed is that the universe can be topologically closed. For example it could have the spatial topology of a three-torus (though a duodecahedron looks more plausible) so space could be flat but there could still be spatially closed worldlines (i.e. you could move in a straight line and return to your starting point). This type of closure is not described by general relativity so we have no way of predicting whether the universe is topologically closed and if so on what scale. However the scale has to be larger than the observable universe otherwise we would have seen evidence for it.
Given that the expansion of the universe is accelerating and will ultimately approach a de Sitter geometry there is no way to return to your starting point even if the universe is topologically closed on some >13.8 billion year length scale.
So while there is some interest in the twin paradox in a topologically closed universe, the question is entirely hypothetical.
