How does the ionic radius of a typical anion compare?
Compared to what, the parent atom?
Well the radius of an anion SHOULD be greater than that of the parent atom. Why? Two factors are important here: (i) nuclear charge; and (ii) shielding by other electrons. These work in opposition; high nuclear charge reduces atomic size inasmuch as it attract the valence electrons; full electronic shells shield the nuclear charge, and force the valence electrons to lie at a greater separation, i.e. a greater radius, from the nuclear core.