Answer
$ Li^{+} \lt Li \lt Li^{-} $
Work Step by Step
In $Li$, 3 electrons are effectively held by 3 protons in the nucleus, so net attraction per electron is 1.
In $ Li^{+}$, 2 electrons are more effectively held by 3 protons in the nucleus, so net attraction per electron is $ \gt 1$. Thus, it's size is less than $Li$.
But for $ Li^{-}$, 4 electrons are more effectively held by 3 protons in the nucleus, so net attraction per electron is $ \lt 1$. Thus it's size is greater than $Li$.
Thus, the size of a cation is way less than its parent atom, while the size of an anion is greater than its parent atom.
Thus the order is, $ Li^{+} \lt Li \lt Li^{-} $
