This article and the science behind it at this point is making some very interesting assumptions, such as that the Universe is closed (like the Earth is in their analogy).
But I believe at this point, the real answer is that we don't yet know. We're looking at a universe that looks flat and basically saying "well, it could be flat and finite, or it could be closed and curved, or it could be flat and infinite... but last time we thought our plane of existence was flat we ended up way off, so we better not make that assumption again."
Point being: take this conclusion about the size of the Universe with a grain of salt.
But I believe at this point, the real answer is that we don't yet know. We're looking at a universe that looks flat and basically saying "well, it could be flat and finite, or it could be closed and curved, or it could be flat and infinite... but last time we thought our plane of existence was flat we ended up way off, so we better not make that assumption again."
Point being: take this conclusion about the size of the Universe with a grain of salt.