  The surprisingly low current density required for inducing the insulator to
metal transition has made Ca$_2$RuO$_4$ an attractive candidate material for
developing Mott-based electronics devices. The mechanism driving the resistive
switching, however, remains a controversial topic in the field of strongly
correlated electron systems. Here we probe an uncovered region of phase space
by studying high-purity Ca$_2$RuO$_4$ single crystals, using the sample size as
principal tuning parameter. Upon reducing the crystal size, we find a four
orders of magnitude increase in the current density required for driving
Ca$_2$RuO$_4$ out of the insulating state into a non-equilibrium (also called
metastable) phase which is the precursor to the fully metallic phase. By
integrating a microscopic platinum thermometer and performing thermal
simulations, we gain insight into the local temperature during simultaneous
application of current and establish that the size dependence is not a result
of Joule heating. The findings suggest an inhomogeneous current distribution in
the nominally homogeneous crystal. Our study calls for a reexamination of the
interplay between sample size, charge current, and temperature in driving
Ca$_2$RuO$_4$ towards the Mott insulator to metal transition.
