# How do you determine increasing ionic size?

And thus we could compare the known structures of say, $\text{lithium chloride,}$ and $\text{sodium chloride,}$ and $\text{potassium chloride.}$ (These are known, to an excellent approximation, by X-ray crystallography experiments). And we find that ionic size INCREASES down a Group, as we would expect. Of course, cations should be intrinsically SMALLER than their parent atoms, and anions should be intrinsically LARGER. Why should this be so for mono-atomic ions?