Fe2+ is greater in size than Fe3+.
Consider the electronic configuration of Fe2+[Ar] 3d6 and Fe3+[Ar] 3d5 (For comparison, Fe [Ar] 4s2 3d6). Fe3+ ionic radius is 63 pm, while Fe2+ has an ionic radius of 77 pm. (For comparison, the Fe atom has a radius of 140 pm). This is because the outermost electron in the Fe2+ ion is pulled off to form Fe3+ ion. As the electrons are removed, it reduces the repulsion increasing nuclear charge experienced by each of the other d electrons and decreases the size of the ion.
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