In given arrangement, the cation is extracted into 4 anions in a plane as shown in figure. The 5th anion is in upper layer (coming outside the paper) and 6th in bottom layer (going in the paper).

Join cation anion centres E and B and complete ΔEBF

In ΔEBF

$m\angle F=90$, and EF=EB

$\therefore m\angle B=m\angle E=45$

where ra is radius of anion and rc is the radius of cation

and EB = ra + rc

and BF = ra

Therefore-

$cosB=cos45=\dfrac{r_a}{r_a}+r_c=0.7071$

$\dfrac{r_c}{r_a}=\dfrac{1-0.771}{0.7071}=0.414$

critical radius ratio of ionic crystal in ligacy 6 configuration is 0.414

When radius of anion is 2.02Å

 $\dfrac{r_c}{2.02}=0.414$

rc = 0.836Å

Maximum size of cation in ligacy 6 configuration is 0.836Å in terms of radius.