When an atom ionizes, either they accept or transfer electrons, their radius changes.
In the formation of cation, the radius of the cation is generally smaller than its atom. For example:
The radius of Na+ is smaller than Na atom
In the formation of anion, the radius of the anion is larger than its atom. For example:
The radius of Cl- is bigger than Cl atom
This happens because when an atom accepts or gives up an electron to form an ion, it generally assumes a noble gas configuration.
For Na, when it loses an electron, its outer shell will be removed thus having a smaller ionic radius. It will have a similar configuration to that of Neon.
In the case of Cl, as it receives another electron, its configuration is like that of Argon, thus increasing its radius.
hope this helps :)