Atomic radius is a measure of the size of an element or atoms of an element..
It increases down the group and decreases across the period..
Cations generally have a smaller radius than their parent atoms, and anions, generally have a larger radius than their parent atoms. To help visualize this:
In more detail, this is mainly due to effective nuclear charge, as electrons are ionized, the effective nuclear charge increase, thus decreasing the radius of the atom. When electrons are added, the effective nuclear charge is less "effective" at holding onto the electrons.