  For a non-empty compact set $E$ in a proper subdomain $\Omega$ of the complex
plane, we denote the diameter of $E$ and the distance from $E$ to the boundary
of $\Omega$ by $d(E)$ and $d(E,\partial\Omega),$ respectively. The quantity
$d(E)/d(E,\partial\Omega)$ is invariant under similarities and plays an
important role in Geometric Function Theory. In the present paper, when
$\Omega$ has the hyperbolic distance $h_\Omega(z,w),$ we consider the infimum
$\kappa(\Omega)$ of the quantity $h_\Omega(E)/\log(1+d(E)/d(E,\partial\Omega))$
over compact subsets $E$ of $\Omega$ with at least two points, where
$h_\Omega(E)$ stands for the hyperbolic diameter of the set $E.$ We denote the
upper half-plane by $\mathbb{H}$. Our main results claim that $\kappa(\Omega)$
is positive if and only if the boundary of $\Omega$ is uniformly perfect and
that the inequality $\kappa(\Omega)\leq\kappa(\mathbb{H})$ holds for all
$\Omega,$ where equality holds precisely when $\Omega$ is convex.
