We know that ionic radii trends have the elements ionic radii increasing right to left in a row and top to bottom in a group with Groups 13-18 having their respective exceptions.
That being said, since Ca2+ and Mg2+ are in the same group, we know that Mg2+ has a smaller radius that Ca2+.
So Mg2+ < Ca2+
And we know that Cl- and S2- are both two large atoms, due to them being found in group 17 and 16 respectively. Both of these atoms have a larger radius than Ca2+ due to them being an exception.
So we can write: Mg2+ < Ca2+ < Cl- and S2-
One way to know the exact figures for sure is to have a table on hand with all of the ionic radii. Using a table, we know that the actual ordering of the atoms is:
Mg2+< Ca2+ < Cl- < S2-
Therefore, Mg2+ has the smallest ionic radius.
