Answer
S $\lt$ Se $\lt$ Sb $\lt$ Pb $\lt$ Cs
Work Step by Step
Atomic size increases as we move from top to bottom and decreases on moving from left to right.
S is present up in the periodic table and on the right side as compared to other elements. Thus it has the smallest size.
Cs is present on the extreme left and is present down in the table, therefore it has the largest atomic size.