Atomic size decreases across a period and increases down a group in the periodic table. For ions, size depends on the number of electrons and the effective nuclear charge. Cations are smaller than their parent atoms. Comparing the given ions:

Rb⁺ is in period 5, so it has the largest number of electron shells. Even though it has a +1 charge, its size is larger than the smaller, more highly charged ions like Ba²⁺ (period 6 but higher charge) or Mg²⁺ (period 3). Li⁺ is the smallest as it is in period 2.

Therefore, Rb⁺ has the largest ionic radius.

Final Answer: Rb⁺