about a tenth of a millionth of a millimetre across (0.0000001 mm).
A piece of dust is exactly half way between the size of the Earth and an atom. Atoms are pretty darn small. Their outer boundaries are set up by moving electrons, so they aren't well defined....
The radius of a sulfur atom is 106 picometers.
The empirical atomic radius of uranium atom is 175 pm.
Like all atoms they are tiny. Compared with other atoms, iron atoms are of medium size.