The size of an ion is determined by its atomic radius, which is a measure of the size of the atom from which the ion is formed. The atomic radius of an atom is generally determined by the size of its electron cloud. The size of the electron cloud is influenced by the number of electrons and the energy levels they occupy.
Of the ions given, Li+ has the smallest atomic radius because it has the smallest number of electrons and the highest effective nuclear charge, which means that its electrons are held more closely to the nucleus. Na+, K+, and CS+ have larger atomic radii than Li+ because they have more electrons and lower effective nuclear charges, which means that their electrons are held more weakly by the nucleus.
Therefore, the answer is CS+ has the largest radius.