  The persistence of the first-order transition line in the phase diagram of
mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ vortex matter is detected down to
a system size of less than hundred vortices. Precise and highly-sensitive to
bulk currents AC magnetization techniques proved to be mandatory in order to
obtain this information. The location of the vortex matter first-order
transition lines are not altered by decreasing the sample size down to 20
$\mu$m. Nevertheless, the onset of irreversible magnetization is affected by
increasing the sample surface-to-volume ratio producing a noticeable
enlargement of the irreversible vortex region above the second-peak transition.
