If we look from the Earth, in opposite directions, at objects that are 13 billion LY away, they are 26 billion LY apart.

We are seeing them as/where they were 13 billion years ago, so they must have been 26 billion LY apart then.

Does this mean that, 13 billion years ago, the Universe was, at the very least, 26 billion LY in diameter?

There never was nothing.