  The afterglows of gamma-ray bursts are non-thermal electron synchrotron
emissions from relativistic shocks. The origin of strong magnetic field in the
emission region remains elusive, and two field amplification mechanisms via the
plasma kinetic and magnetohydrodynamic instabilities have been discussed. The
polarimetric observations are a powerful probe to distinguish these two
mechanisms. So far, most theoretical works have focused on the former mechanism
and constructed afterglow polarization models with microscopic-scale turbulence
whose coherence length is much smaller than the thickness of the blast wave. In
this work, focusing on the latter mechanism, we utilize our semi-analytic model
of the synchrotron polarization with large-scale turbulence whose coherence
length is comparable to the thickness of the blast wave to investigate the
effect of magnetic field anisotropy and the observer viewing angle. We find
that the polarization in our large-scale turbulence model can exhibit both
behaviors characteristic of the microscopic-scale turbulence model and those
not seen in the microscopic-scale model. Then we find that the large-scale
model could explain all the polarimetric observational data to date that seem
to be forward shock emission. We also examine the effect of ordered-field
component, and find that polarization degree and polarization angle constant in
time are realized only when the energy density ratio of the ordered and
fluctuated components is $\gtrsim 50$. In this case, however, the polarization
degree is much higher than the observed values.
