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Yes, it could shrink, and it could stop expanding too. Not because of "some sort of barrier" like a balloon expanding inside a room and hitting the walls, but because of some inherent "potential barrier", a critical point where the process might reverse, like a pendulum's swing reaching its highest point, stopping, and reversing direction.
There is no external space in which the universe is doing its expanding. It is space itself that is expanding. The distance between any two points is increasing over time, that is all. Whether space is finite or infinite has nothing to do with this. And if it is finite, it is more in the sense of a circle having a finite circumference even though you could traverse it round-and-round for ever (i.e., finite but no boundary), and not in the sense of a straight ruler which has a finite length because it has two end points a finite distance away, which you can go beyond and step off the edge.
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