In the isoelectronic species the ionic radii  $\left({\mathrm{A}}^{°}\right)$ of ${\mathrm{N}}^{-3},{\mathrm{O}}^{-2},{\mathrm{F}}^{-}$are respectively given by

 For the iso electronic species  ${\mathrm{N}}^{-3},{\mathrm{O}}^{-2},{\mathrm{F}}^{-},$more the negative charge Present on the ion. Hence they have more ionic radii. 

For ${\mathrm{N}}^{-3},{\mathrm{O}}^{-2},{\mathrm{F}}^{-},$ ionic radii's $1.71,1.4,1.36$

Note:   If the net charge on anion increases then the size will increase for isoelectronic species.