  We report the measurement of the triplet s-partial-wave scattering length $a$
of two bosonic isotopes of the highly magnetic element, dysprosium:
$a=122(10)a_0$ for $^{162}$Dy and $a=92(8)a_0$ for $^{164}$Dy, where $a_0$ is
the Bohr radius. The scattering lengths are determined by the cross-dimensional
relaxation of ultracold gases of these Dy isotopes at temperatures above
quantum degeneracy. In this temperature regime, the measured rethermalization
dynamics can be compared to simulations of the Boltzmann equation using a
direct-simulation Monte Carlo (DSMC) method employing the anisotropic
differential scattering cross section of dipolar particles.
