  We investigate the scaling properties in inclusive hadron production and the
associated single transverse spin asymmetry in the forward rapidity region at
RHIC. We find that the spin-averaged experimental data in both $pp$ and $dAu$
collisions demonstrates a transverse-momentum-dependent geometric scaling. We
introduce the transverse momentum dependent Collins fragmentation function to
study the scaling of the single transverse spin asymmetries. The general
feature of the scaling analysis is consistent with the experimental
observations, in particular, for the transverse momentum dependence of the spin
asymmetries at RHIC energy. We further propose to probe the saturation scale of
nucleus by measuring the spin asymmetry normalized by that in $pp$ scattering
at low transverse momentum.
