Finding a minimal context free grammar that recognizes a finite set of strings of bounded length
If $\ell$ and $k$ are fixed, there are only finitely many possible problem instances, so you can write a program that has a hardcoded table of all possible instances and their solutions.  Consequently, the complexity will be $O(1)$, if you consider $\ell$ and $k$ as fixed and look at the asymptotics as $n$ increases.

Why are there only finitely many possible problem instances?  Because when $\Sigma$ and $\ell$ are fixed and $\Sigma$ is finite, there are only finitely many subsets of $\Sigma^1 \cup \Sigma^2 \cup \Sigma^3 \cup \cdots \cup \Sigma^{\ell}$.