  We determine the spin wave exchange stiffness $D$ and the exchange constant
$A$ for the full Heusler compound \CFS using Brillouin light scattering
spectroscopy. We find an extraordinarily large value of $D=715\pm20$ meV
\AA$^2$ ($A=31.5\pm1.0$ pJ/m) which is, to the best of our knowledge, only
surpassed by the intermetallic compound Fe$_{53}$Co$_{47}$ (J. Appl. Phys.
\textbf{75}, 7021 (1994)). Furthermore, we provide a systematization of the
exchange stiffnesses determined for a variety of Co$_2$-based Heusler
compounds. We find that for the investigated compounds, the exchange stiffness
is a function of the valence electron concentration and the crystallographic
ordering. The exchange stiffness increases when the valence electron
concentration and/or the amount of the L2$_1$ ordering increase. A qualitative
explanation for the dependence on the valence electron concentration is
provided.
