A harder question would be ... predicting the ion's radius.
I always forget this trend. Somebody help me if you know the answer.
K+ < Ar < Cl- <S2-
Next... the reasoning behind this is that they all have the same number of electrons but different number of protons. Because sulfur has the least number of protons attracting the same number of electrons, its outermost valence electrons feel less pulling force and thus orbit in a larger circle than the rest.