What is the relative size of the radius of a negative ion to its neutral atom?
It is larger.
In general, any negative ion is formed by the acquisition of additional electrons in its valence orbitals. That implies a specific, if minute, increase in the “electron distribution cloud”. It also increased the average orbital size because the same number of protons are “holding on to” a larger number of electrons.
See also the following page for a detailed discussion, diagrams and charts:
This is the interactive site from which the data were obtained: http://abulafia.mt.ic.ac.uk/shannon/ptable.php