Cesium has the largest ionic radius of these four elements. Ionic radius is a trend that increases going down in a group of the periodic table. These ions are all in the alkali metals family, and their order both from top to bottom and from small to large is:
Li+, Na+, K+, Cs+
Each of these has lost its one valence electron, leaving the energy level below complete. For example, sodium lost its 3s1 electron and now ends in 2p6. They all have the same number of outer or valence electrons. However, they don't have the same number of inner electrons. As you move down in a group each element has more inner electrons than the one before it. Inner electrons shield the outer electrons from some of the attraction of the nucleus because electrons repel other electrons. This causes the outer electrons to be less attracted and stay farther from the nucleus, resulting in a larger atom or ion.