How big is the universe?
We can see about 93 billion light-years across, and the whole thing is likely far larger, maybe infinite.
What we can actually see has an edge
The universe has a finite age, about 13.8 billion years, and light travels at a finite speed. So there is a horizon: a sphere around us holding everything whose light has had time to reach us. That sphere is the observable universe, and it has a hard edge for one boring reason: not enough time has passed for anything farther away to be seen.
Why the edge sits farther than you'd expect
Here is the counterintuitive part. If the universe is only 13.8 billion years old, how can we see things 46 billion light-years away? Because space is not a fixed stage, it stretches. Light that set out long ago has been riding an expanding cosmos, and the galaxies that emitted it have drifted much farther out in the meantime.
And beyond the horizon?
The observable universe is not the whole universe, only the part we can receive signals from. Measurements of the cosmos's geometry show it is flat to within tiny error, which hints the true extent is enormously larger than what we see, and possibly without any edge at all.
“Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is.”
- Douglas Adams A joke that undersells the actual numbers, the observable universe alone holds an estimated two trillion galaxies.
So the honest answer comes in two parts. The part we can observe is about 93 billion light-years wide. The whole of it, that we cannot measure, and it may be infinite.