In an isoelectronic series, a greater nuclear charge results in a smaller ionic radius. Consequently, the ionic radii of the given isoelectronic species decrease in the following order, corresponding to increasing nuclear charge: \[ \text{S}^{2-} > \text{Cl}^- > \text{K}^+ > \text{Ca}^{2+}. \] \( \text{S}^{2-} \) possesses the smallest nuclear charge, while \( \text{Ca}^{2+} \) has the largest.