The atomic radius of an element is the distance from the nucleus to the outermost electron shell. It generally decreases across a period (row) from left to right and increases down a group (column) in the periodic table.

Among the given elements, chlorine (Cl) has the smallest atomic radius. The atomic radii of bromine (Br), sulfur (S), and selenium (Se) are larger than that of chlorine. The order of atomic radii is:

Cl < S < Se < Br