I think the smallest computer that can simulate the universe is the universe. Though I guess you may be able to get rid of one of the dimensions due to that one projection theory. Which means you may be able to get ride of more than one dimension. Which means maybe the universe can fit into a single infinitely dense point. So maybe we can make black hole computers. We’d just need to bend space time in a real specific way because what’s the point of a computer you can’t get any output from?

tl;Dr: I bet we could figure out how to simulate a whole universe within a decently small computer. Seems hard though.