Atomic size decreases across a period and increases down a group. Cations are smaller than their parent atoms. Among isoelectronic ions, size decreases with increasing nuclear charge.

Comparing the given species:

• Na (sodium atom) is the largest.
• Na⁺ and Li⁺ are cations, smaller than their atoms.
• Na⁺ and Li⁺ are isoelectronic (both have 10 electrons).
• Li⁺ has a higher nuclear charge (Z=3) than Na⁺ (Z=11) for the same number of electrons, pulling the electron cloud closer.

Therefore, Li⁺ has the smallest ionic radius.

Final Answer: Li⁺