Isoelectric species are simply the atoms or ions that have same number of electrons. For example:- Na+ , Ne , F- , O²- , Mg²+
And their size depends upon the effective nuclear charge , in case of cations the number of protons is higher than that of electrons thus their size is smaller than that of neutral atom.
For example the size of Na+ is smaller than neutral Na.
And further size of Mg²+ is smaller than Na+ due to more nuclear charge.
But if we talk about anion the size increases due to the increased number of electrons and the nuclear charge can not attract the electrons effectively thus making the electron cloud to expand. And thus size increase.
For example in case of size of F- is larger than nutral F .
And the size of O²- is larger than F- .