  The insulator terminating the fractional quantum Hall series at low Landau
level filling \nu is generally taken to be a pinned Wigner crystal (WC), and
exhibits a microwave resonance that is interpreted as a WC pinning mode.
Systematically studying the resonance in a high quality sample for carrier
densities, n, between 1.8 and 5.7 x 10^{10} cm^-2, we find maxima in resonance
peak frequency, f_pk, vs magnetic field, B. L, the domain size, or correlation
length of Wigner crystalline order, is calculated from f_pk. For each n, L vs
\nu tends at low \nu toward a straight line with intercept; the linear fit is
accurate over as much as a factor of 5 range of \nu. We interpret the striking
linear behavior as due to B compressing the electron wavefunctions, to alter
the effective electron-impurity interaction.
