  We investigate the critical current, $I_C$, of ballistic Josephson junctions
made of encapsulated graphene/boron-nitride heterostructures. We observe a
crossover from the short to the long junction regimes as the length of the
device increases. In long ballistic junctions, $I_S$ is found to scale as
$\propto \exp(-k_bT/\delta E)$. The extracted energies $\delta E$ are
independent of the carrier density and proportional to the level spacing of the
ballistic cavity, as determined from Fabry-Perot oscillations of the junction
normal resistance. As $T\rightarrow 0$ the critical current of a long (or
short) junction saturates at a level determined by the product of $\delta E$
(or $\Delta$) and the number of the junction's transversal modes.
