  We demonstrate a geometrical effect on the depinning line (DL) of the flux
line lattice of the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ high-T$_c$
superconductor (HTSC) micrometer ring. The DL shifts to notably lower
temperatures in comparison to bulk crystals and thin flakes of the same sample.
The shift is attributed to a decrease in the overall pinning potential due to a
double size effect, namely: a) the ring thickness $\sim 1~\mu$m being smaller
than the pinning correlation length, and b) the increase in the effective
London penetration depth of the vortices (Pearl vortices). The large shift of
the DL to lower temperatures may influence the suitability of this HTSC for
applications in microstrip antennas and THz emitters.
