I think both are quite huge. I believe the power of immagination is great, but it has limits, as most of what we know is acquired by the senses, and people cannot think about things they can't perceive...you cannot immagine a colour you've never seen.
As for the universe, it's quite big too...and we still don't know for sure whether it has boundaries, do we?
Anyway, I think the universe is bigger...maybe because the minds are part of it.