How does ionic radius change down a group in the periodic table?
Both atomic size and ionic size INCREASE DOWN a Group.
The increase is inevitable. You are filling successive valence shells of element going down a Group. Of course, you are removing valence electrons upon ionization, but you build on successively expanded shells of core electrons. As a chemist, as a physical scientist, you should consult some physical data, and quantify the change. Try the wiki site.