The insulator terminating the fractional quantum Hall series at low Landau level filling ν 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

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... ted from f_pk. For each n, L vs ν tends at low ν toward a straight line with intercept; the linear fit is accurate over as much as a factor of 5 range of ν. We interpret the striking linear behavior as due to B compressing the electron wavefunctions, to alter the effective electron-impurity interaction.