electronic conf. of cs + is (2,8,18,18,8) .it has 5 atomic shells.so.the effective nuclear charge on each electron is less.the screening effect is less ,so the size is less.whereas in cl minus ,it has 3shells ,so the effective nuclear charge on each electron is more.but the size of it is greater due to the inter-electronic repulsion in the atom .so the size of ci minus is greater than cs+.

i can give you another example of inter -electronic repul.check out the atomic sizes of cl &Ar.according to the atomic size theory ,size of Ar  should be more than cl coz' of less electrons in cl.compare these two cases and you will get the concept.i don't expect you to rate me.

Ionic radius of Cs⁺ = 1.67A⁰

Ionic radius of Cl ^-  = 1.84A⁰

In Cs, when the outer electron is removed to give positive ion Cs⁺, the size decreases considerably . This is so because the outermost shell of electrons has been removed completely, and having removed an electron, the positive charge on the nucleus is now greater than the charge on the remaining electrons, so that each of the remaining electrons is attracted more strongly towards the nucleus.

In Cl ^- , due to addition of an electron, the negative charge increases , and the electrons are attracted less strongly towards the nucleus. This increases the size.