If the Milky Way galaxy is as big as planet earth, how big is Earth? If the Milky Way galaxy is as big as planet earth, how big is Earth?
Well, they are different shapes, but we can get a rough answer by doing a ratio calculation based on their radii.

Let $g$ be the radius of the galaxy, about $5\times 10^{20}\ \mathrm m$.
Let $e$ be the radius of the Earth, about $6371\ \mathrm{km}$.
Let $s$ be the radius of the scaled-down Earth. 

Then,
$$\frac e g = \frac s e$$
So $$s = \frac{e^2} g = \frac {(6371\ \mathrm{km})^2} {5\times 10^{20}\ \mathrm m}$$
which is approximately 80 nanometres. So it's about 1000 times the size of an atom.