The distance between the centre of nucleus to the electron in valence orbit is called ionic radii  anion has negative charge  that is addition of electron. so now number of protons are less than number of electrons  so due to this some replusive forces arises due unbalanced charge results in increase in size  where as in parent atom number of protons =number of electrons
IONIC RADIUS - It is the distance between the centre of the nucleus and the electron in the outermost shell(valence shell).

The size of an atom is maintained when no. of protons are equal to the no. electrons because protons are positively charged and electrons are negatively charged and thus protons bound electrons with a force(called nuclear force) which effects the size of an atom. But, in anions, electrons are gained by the parent atom which causes a decrease in the nuclear charge(because no. of electrons become more than no. of protons) and thus the atomic size of anion becomes more than that of parent atom.

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