which is greater in size:-

Fe2+ or Fe3+?

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 the 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.