Answer
176 pm
Work Step by Step
There's one ion each in the unit cell so:
Mass: $(132.905+35.45)\ g/mol\div 6.022\times 10^{23}\ atoms/mol=2.796\times10^{-22}\ g/cell$
Volume: $2.796\times10^{-22}\ g/cell\div 3.99\ g/cm^3=7.007\times 10^{-23}\ cm^3/cell$
Side length: $\sqrt[3]{7.007\times 10^{-23}\ cm^3}=4.12\times10^{-8}\ cm=412\ pm$
Cube diagonal: $l.\sqrt3=2.r_{Cs^+}+2.r_{Cl^-}$
$r_{Cs^+}=176\ pm$
