The unit cell cube length for LiCl(just like NaCl structures) is `5.14Å`. Assuming anion-anion contact, the ionic radius for chloride ion is:

Explanation

In a face centred cubic lattice, anions touch each other along the face diagonal of the cube
`4r_(Cl^(-))=sqrt(2)xx a`
`therefore r_(Cl^(-))=(sqrt(2))/(4)xx a`
`= (sqrt(2))/(4)xx 5.14`
`= 1.82 Å`
Alternatively
Distance between `Li^(+)` and `Cl^(-)` ion `= (5.14)/(2)= 2.57 Å`
Thus distance between two chloride ions
`= sqrt((2.57)^(2)+(2.57)^(2))=3.63 Å`
`therefore r_(Cl^(-))=(3.63)/(2)=1.82 Å`