Periodic Table — Periodic Properties & Variation of Properties — Question 17

Atomic radii of an atom increases on moving down a group. As we move down a group, the number of shells increases and the nuclear charge also increases. But increase in the number of shells dominates over increase in nuclear charge therefore, overall atomic size increases.
For alkali metals in Group 1 [IA], Li which is at the top of the group has the smallest atomic radius whereas Cs at the bottom of the group has the largest atomic radius. The atomic radii of the elements has the following relation:

Li < Na < K < Rb < Cs