AP EAMCET Engineering 21 Apr 2019 Shift 1 Paper

Z represents number of protons in a nucleus. Thus, for an isoelectronic species, Size of anion > atom > cation.
It means the species with more electrons w.r.t. protons in nucleus is of larger size that is, has smaller value of Z Therefore, Z−2has least number of protons, whereas Z−1 has maximum number of protons.
Thus, the order of size for isoelectronic species is Z−2>Z>Z+1