Atomic radius decreases on going from left to right in a period. Thus size of $\mathrm{O}>\mathrm{F}$ and size of anion is larger then their atoms. As ${\mathrm{O}}^{2-}$ and ${\mathrm{F}}^{-}$ are isoelectronic, therefore, size of ${\mathrm{O}}^{2-}>{\mathrm{F}}^{-}.$