Cations is smaller & Anions is bigger than parent atom.

Cations is formed by the loss of electrons, hence proton are more than electron in a cation  co electron are strongly attached  by the nucleus and are pulled inward hence size decrease.
E.g. Na - e- > Na+

on the other hand anion is larger than the parent atom. because anion are formed by the gain of electron. thus the number of electron are more than proton . the effective positive charge in the nucleus is less , so less inward pull is experienced. hence the size is expand.

Cl + e -  > Cl-

Thats it.