Answer
$K^+$ ion is smaller than $Cl^-$ ion.
Work Step by Step
Both $Cl^-$ and $K^+$ have 18 electrons, so their electron-electron repulsions are quite the same.
However, $Cl^-$ has a nuclear charge of $17+$, while $K^+$ has a nuclear charge of $19+$. Therefore, the attraction of the $K^+$ nucleus is greater than that of the $Cl^-$ nucleus.
That means the electrons of $K^+$ are more attracted and closer to the nucleus than those of $Cl^-$. Therefore, $K^+$ ion is smaller than $Cl^-$ ion.
