Na+.Mg2+,Al3+ and Si4+ are isoelectronic. The order of their ionic size is
Na+>Mg2+>Al3+>Si4+
In isoelectronic species the number of electrons are same but nuclear charge is different. As the nuclear charge increase, the attraction force on last electron increases, so the size decreases or in other words
Ionic size∝1Charge on cation and hence, order is
Mg2+>Al3+>Si4+−−−−−−−−−−−→
Nuclear charge increase−−−−−−−−−−−−−−−→Size decrease