Answer
$Na^+$ ion is smaller than $F^-$ ion.
Work Step by Step
Both $F^-$ and $Na^+$ have 10 electrons, so their electron-electron repulsions are quite the same.
However, the nuclear charge of $F^-$ is $9+$, while the nuclear charge of $Na^+$ is $11+$. That means the attraction of the $Na^+$ nucleus is greater than that of the $F^-$
Therefore, the electrons of $Na^+$ are more attracted and closer to the nucleus than those of $F^-$. As a result, $Na^+$ ion is smaller than $F^-$ ion.
