The order of magnitude of ionic radii of ions ${\mathrm{Na}}^{+},{\mathrm{Mg}}^{2+},{\mathrm{Al}}^{3+}\mathrm{and}{\mathrm{Si}}^{4+}\mathrm{is}$

${\mathrm{Na}}^{+}>{\mathrm{Mg}}^{2+}>{\mathrm{Al}}^{3+}>{\mathrm{Si}}^{4+}$ .  All are isoelectronic but nuclear charge per electron is greatest for ${\mathrm{Si}}^{4+}.$ So it has smallest size and nuclear charge per electron for ${\mathrm{Na}}^{+}$ is smallest. So it has largest size.