  We study finite-size effects at the onset of the irreversible magnetic
behaviour of micron-sized Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals by
using silicon micro-oscillators. We find an irreversibility line appearing well
below the thermodynamic Bragg-glass melting line at a magnetic field which
increases both with increasing the sample radius and with decreasing the
temperature. We show that this size-dependent irreversibility line can be
identified with the crossover between the Larkin and the random manifold
regimes of the vortex lattice transverse roughness. Our method allows to
determine the three-dimensional weak collective pinning Larkin radius in a {\it
direct way}.
