The ionic size decreases in the order

${\mathrm{S}}^{2-},{\mathrm{K}}^{+},{\mathrm{Sc}}^{3+},{\mathrm{V}}^{5+}$and ${\mathrm{Mn}}^{7+}$ have equal no. of electrons. So, they are iso-electronic. For iso-electronic species, size is inversely proportional to the effective nuclear charge. Effective nuclear charge is directly proportional to the positive charge. 

Thus more the positive charge less the size. Therefore, order of size is :${\mathrm{S}}^{2-}>{\mathrm{K}}^{+}>{\mathrm{Sc}}^{3+}>{\mathrm{V}}^{5+}>{\mathrm{Mn}}^{7+}$