Atom & Ion Size Comparison:

1) For cation and its original atom, the cation is smaller than its original atom in size.  Ex. Radius of Fe2+ is smaller than radius of Fe.

Reason is there is less electrons in the cation, making the shielding effect from the electrons smaller while the attraction force from the nucleus remains the same.

 

2) For anion and its original atom, the anion is greater than its original atom in size.   Ex. Radius of Cl- is greater than radius of Cl.

Reason is there is more electrons in the anion, making the shielding effect from the electrons stronger while the attraction force from the nucleus remains the same.