If you could shrink the universe down to the size of earth, how big would earth be?

It's dangerous.

Why? Because if you can shrink the universe to the size of the Earth, regarding all the fundamental forces and behaviour of elementary particles, It would, theoretically ,collapse into a black hole.

But, in terms of scale, if you can shrink the whole universe (not just the observable universe) ,to the size of the Earth, the Earth would be nothing compared to the size of the universe. The Earth, would be incredibly small, in that scale, our planet would be smaller than a fundamental particle. So you cannot say that the earth still Exists or still there.