If the whole observable Universe is as big as planet Earth, how big would Earth be?
The radius of the observable Universe is 41.5billion light years (ref).
A light year is $9.5\cdot10^{12}$ km.
The radius of the Earth is 6371 km.


So the answer is $6371 km \cdot \frac{6371 km}{41.5\cdot10^9\cdot9.5\cdot10^{12}km} = 1.03\cdot10^{-16}km = \underline{\underline{1.03\cdot10^{-13}m}}$.

The size of the atoms are $\approx 10^{-10}m$.