The atomic radius of an atom is the radius from the nucleus in which 90% of the electron density is contained.

Atomic Radius of Ions

For any given element, it's cation will have a smaller atomic radius than the neutral species. This is because with fewer electrons the effective nuclear charge increases causing the electrons to be more tightly bound to the nucleus.

Conversely, the anion always has a larger atomic radius than the neutral species. Again this is because of the effective nuclear charge which decreases as electrons are added causing the electrons to be less tightly bound to the nucleus.