  We investigate the attenuation law in $z\sim 6$ quasars by combining
cosmological zoom-in hydrodynamical simulations of quasar host galaxies, with
multi-frequency radiative transfer calculations. We consider several dust
models differing in terms of grain size distributions, dust mass and chemical
composition, and compare the resulting synthetic Spectral Energy Distributions
(SEDs) with data from bright, early quasars. We show that only dust models with
grain size distributions in which small grains ($a < 0.1~\mu$m, corresponding
to $\approx 60\%$ of the total dust mass) are selectively removed from the
dusty medium provide a good fit to the data. Removal can occur if small grains
are efficiently destroyed in quasar environments and/or early dust production
preferentially results in large grains. Attenuation curves for these models are
close to flat, and consistent with recent data; they correspond to an effective
dust-to-metal ratio $f_d \simeq 0.38$, i.e. close to the Milky Way value.
