In this question shouldnt edge length be
2(radius of cl)+(radius of cs+) divided by root 3
CsCl is BCC cubic lattice packing
in this case edge length (a) = 4r/ (3)^1/2
But 4r = 2r(Cl-) +2r(Cs+)
use above equation and plugin given values of Cs+ ion and Cl- radii given in the question.