When a metal forms a cation, losing one or more electrons, the size of the resulting ion is less than that of the neutral atom. When a nonmetal gains an electron form X^-, the ion will be greater in size than the neutral atom. This is due to the increase in repulsion among the electrons.
a. Fe or Fe^3+ .... Fe^3+ is smaller
b. Na or Na^1+ ... Na+ is smaller
c. N or N^3- ........ N is smaller .... N^3- would be larger if it existed, which it doesn't (*)
d. I or I^1- ........... I is smaller
e. Al or Al^3+ ...... Al^3+ is smaller
The "nitride ion" has an oxidation state of -3, but it does not have an actual charge of -3. The oxidation state is the hypothetical charge on an atom or ion if all of the bonds are 100% ionic. Of course we know that there are no 100% ionic bonds, and compounds which form metal nitrides have bonds with some or even significant covalent character. It is important to understand that the oxidation state is not the actual charge on an atom in a compound.
The only stable alkali metal nitride is Li3N. Most of the alkaline earth metals form nitrides as do some of the transition metals, but these have more covalent character than ionic character.
The nitride ion would never be found in aqueous solution because it is far to basic and would react with water.
Li3N(s) + 3HOH(l) --> 3LiOH(aq) + NH3(g)