No one knows for sure, but we can make estimates based on the expected age of the universe and the motions of galaxies throughout the universe. The universe itself is expanding, but not in the way a balloon expands. The expansion is taking place throughout the universe, where space-time itself is being stretched outwards. Whereas a balloon pushes its edges out as it expands, the universe is also pushing its insides outwards as well, but there is no centre of the universe, so everything is moving away from everything else. It’s a bit like baking a ball of dough; the entire dough expands and grows, not just its edges.
However, based on our knowledge of how old the universe is, roughly 14 billion years, we can observe a theoretical ‘edge’ of the visible universe about 14 billion light years away from us. This is the furthest distance we can see, as light that might be further away has not had time to reach us yet. Thus, we can say that the visible universe has a diameter of 28 billion light years, 14 billion light years in either direction from Earth. However, what’s beyond this distance is unknown. The 28 billion diameter visible universe we can observe could be just a tiny fraction of a much larger universe, or perhaps just one in a system of many universes.