  We investigate the finite-size scaling behavior of the conductivity in a
two-dimensional Dirac electron gas within a chiral sigma model. Based on the
fact that the conductivity is a function of system size times scattering rate,
we obtain a two-parameter scaling flow toward a finite fixed point. The latter
is the minimal conductivity of the infinite system. Depending on boundary
conditions, we also observe unstable fixed points with conductivities much
larger than the experimentally observed values, which may account for results
found in some numerical simulations. By including a spectral gap we extend our
scaling approach to describe a metal-insulator transition.
