My guess is it’s not really constrained at either end, there actually is no such thing as big or small, the universe is scale-free and everything is relative.

Roger Penrose has suggested as the universe spreads out and you get a very large, boring, empty universe it will eventually become equivalent mathematically to a very small, dense universe and another “Big Bang” will occur, not because the universe got small but because the big started looking the same as the small.

Or something.