It is not small:
Observations from the Sloan Digital Sky Survey and the Planck satellite are where we get the best data. They tell us that if the Universe does curve back in on itself and close, the part we can see is so indistinguishable from “uncurved” that it must be at least 250 times the radius of the observable part.
This means the unobservable Universe, assuming there’s no topological weirdness, must be at least 23 trillion light years in diameter, and contain a volume of space that’s over 15 million times as large as the volume we can observe. If we’re willing to speculate, however, we can argue quite compellingly that the unobservable Universe should be significantly even bigger than that.