A remark on the word length in surface groups

Let $Sigma$ be a surface of negative Euler characteristic and $S$ a generating set for $pi_1(Sigma,p)$ consisting of simple loops that are pairwise disjoint (except at $p$). We show that the word length with respect to $S$ of an element of $pi_1(Sigma,p)$ is given by its intersection number with a well-chosen collection of curves and arcs on $Sigma$. The same holds for the word length of (a free homotopy class of) an immersed curve on $Sigma$. As a consequence, we obtain the asymptotic growth of the number of immersed curves of bounded word length, as the length grows, in each mapping class group orbit.