  Motivated by the recent discoveries of magnets harboring short-pitch skyrmion
lattices, we investigate the skyrmion-size dependence of the topological Hall
effect. By means of large-scale real-space calculations, we find that the Hall
conductivity takes its extreme value in the crossover region where both the
real-space and momentum-space Berry curvature play a crucial role. We also
investigate how the optimum skyrmion size ($\lambda_{\rm sk}^*$) depends on the
lifetime of itinerant electrons ($\tau$) and coupling constant between
electrons and localized spins ($J$). For the former, we show that $\lambda_{\rm
sk}^{*}$ is proportional to $\sqrt{\tau}$, which indicates that $\lambda_{\rm
sk}^{*}$ is much less sensitive to $\tau$ than the conventional expectation
that $\lambda_{\rm sk}^{*}$ is proportional to the mean-free path $\propto
\tau$. For the latter, we show that the non-adiabaticity considerably
suppresses the topological Hall effect when the time scale determined by the
skyrmion size and Fermi velocity is shorter than $1/J$. However, its effect on
$\lambda_{\rm sk}^{*}$ is not so siginificant and $\lambda_{\rm sk}^{*}$ is
about ten times the lattice constant in a wide range of $J$ and $\tau$.
