  Accretion flows onto supergalactic-scale structures are accompanied with
large spatial scale shock waves. These shocks were postulated as possible
sources of ultra-high energy cosmic rays. The highest particle energies were
expected for perpendicular shock configuration in the so-called "Jokipii
diffusion limit", involving weakly turbulent conditions in the large-scale
magnetic field imbedded in the accreting plasma. For such configuration we
discuss the process limiting the highest energy that particles can obtain in
the first-order Fermi acceleration process due to finite shock extensions to
the sides, along and across the mean magnetic field. Cosmic ray outflow along
the shock structure can substantially lower (below ~10^18 eV for protons) the
upper particle energy limit for conditions considered for supergalactic shocks.
