Sakyamuni has said: It is so large that it has no exterior, and so small that it has no interior.
It means that at the Tathagata level, in terms of largeness, you cant see the edge of the universe, and in terms of smallness, you cant see the smallest microscopic particle of matter.
Sakyamuni has also talked about the theory of 3,000 boundless universes. His opinion was that in our Milky Way galaxy (our universe) there are 3,000 planets that have beings.
He has also added that a grain of sand has 3,000 boundless universes in it, so a grain of sand is like universe.