Answer
The size of the atoms from largest to smallest is: Cs, K and Li.
Work Step by Step
The size of an atom depends on the atomic radius of that atom. In the periodic table, there are two trends in bonding atomic radii:
1) From left to right across a period, the bonding atomic radii of the atoms tend to become smaller and smaller (due to the increasing effective nuclear charge).
2) From top to bottom across a group, the bonding atomic radii of the atoms tend to increase (due to the expansion of the principal quantum number $n$, giving the electrons more chance to be further from the nucleus)
K, Li and Cs are in the same group 1A. As mentioned above, in the same group, the bonding atomic radii tend to increase from top to bottom.
From top to bottom in group 1A, the elements appear in the following order: Li, K and Cs.
That means the bonding atomic radii from largest to smallest, and also the size of the atoms is: Cs, K and Li.
