$KI + I _2 \rightarrow KI _3$

The shape of $KI _3$ is determined as the tri-iodide ion $I _3{ }^{-}$ and $K ^{+}$exist linearly.

Here, shape of tri-iodide ion is:

Lone pair $=3$

bond pair $=2$

Hybridization $= sp ^3 d$

structure $=$ Trigonal bipyramidal

Shape $=$ linear (two bond pair only)

Hence, shape is linear