What happens to ionic size across a period?
Atomic size decreases across the Period from left to right. Ionic size, should increase from left to right.
We know that partly filled electronic shells shield nuclear charge very imperfectly. Atomic size thus decreases across the Period from left to right.
However, ionic size should increase across the Period from left to right. Why? Because the atoms of the LHS of the Table (as we view it) are METALS, which are reducing species (and therefore get oxidized), whereas atoms on the right hand side of the Periodic Tables are oxidizing species, electron acceptors, and therefore get reduced. The ionic size of fluoride and oxide anions should be much greater than their parent atoms, because they have extra electronic charge to accommodate.
In these discussions of reactivity we can reasonably ignore the chemistry of the Noble Gases, which have a complete electronic shell.