What we have is an idea of how large the OBSERVABLE universe might be, but really, what does it look like to someone on the edge of that universe? Do you see galaxies on one side of you and nothingness on the other? Or does the Universe's own gravity bend light around itself so that you see shit all around you and have no idea that you're on the "edge"?
it would be extremely dark, since the edge of the universe would have very little visible light (most of its edge being high-energy particles released during the big bang). stay in one place long enough, and you'd eventually see something, but then you wouldn't be at the edge of the universe anymore!
Additionally you can't simply discount the question of where the universe ends as a meaningless question based on the rationale that spacetime occupied by the universe defines the borders of where something CAN exist, because if the Universe exists and can expand to occupy more space then there must already be space for it to occupy, right?
No, you really can discount it!
The problem is, we're too hung up on thinking about things, so even nothing ends up having some physical parameters; a box full of nothing is bounded within a certain space. Even a vacuum in space has a boundary that can be measured by travelling through it.
but the universe's shape is defined by the contents of the universe. whatever the furthest point of the universe is, that point will be occupied by matter and/or energy. and if you ask "what's beyond that point?", it's really nothing... because there's actually nothing there to define the spacetime.
so, to prove me wrong, you travel to the edge of the universe, and then go BEYOND the point. You say "see? there WAS something beyond the edge of the universe!" but now, YOU are the thing defining the edge of the universe.
What's beyond you? Throw a baseball - now it defines the edge.
And so on, and so on, and so on. Turtles, all the way down.