The size of isoelectronic species-${\mathrm{F}}^{-}$, $\mathrm{Ne}$ and ${\mathrm{Na}}^{+}$is affected by :

Nuclear charge has an effect on the sizes of the isoelectronic species ${\mathrm{F}}^{-}$, $\mathrm{Ne}$, and ${\mathrm{Na}}^{+}$(Z). The attraction between the nucleus and the valence electrons will be stronger the higher the nuclear charge, and the ionic size will be smaller. As a result, ${\mathrm{F}}^{-}$ has the largest ionic size while ${\mathrm{Na}}^{+}$ has the smallest.