The problem with the Universe is we’re still not quite sure what the shape of it is. That might sound like a weird question, and hopefully, some day, you’ll study cosmology and be able to explain to me what the answer is, because I don’t fully understand it myself.
But let me try and explain. The Universe can have 3 different “shapes”. We call these shapes “open”, “closed” or “flat”. If our Universe is “open”, then it is infinite, and we won’t be able to judge its size. If our Universe is “closed”, then the Universe is finite, and we should be able to measure how big it is. If the Universe is “flat” then…it can be either infinite or finite! So we won’t be able to tell :O
Right now, our best measurements are telling us that the Universe is “flat”. Whatever that means 😛