  I introduce a simple empirical measure of average galaxy cluster sizes based
on a comparison of the correlation lengths of red galaxies with blue that can
provide a more accurate and bias-free measure of the average size and number
density of galaxy clusters. Using 269,000 galaxies from the SDSS DR6 survey, I
show that this 3D correlation length, averaged over many clusters, remains very
nearly constant at L0= 4.797 +/- 0.024 Mpc/h from small redshifts out to
redshifts of 0.5. This serves as a new measure of cosmic length scales and
provides a means of testing the standard cosmological model that is almost free
of selection biases. The unprecedented accuracy of the technique allows the
possibility of sensitive searches for large-scale inhomogeneities. Applying the
same technique to the Millennium Simulation galaxies I find very good agreement
between it and the SDSS galaxies.
