Transverse Single Spin Asymmetries (TSSAs) manifest themselves through
an azimuthal-angle dependence of particle production relative to the
transverse spin direction of the polarized proton in the reaction
p↑ + p → h + X. In recent years, attention has been given to the interplay
with small-x physics by studying TSSAs in transversely-polarized proton-ion
collisions (p↑+A). Recent calculations of TSSAs in p↑+A collisions predict
that they can be sensitive to the nuclear size (or mass number A) and the
saturation scale (Q_s). The PHENIX experiment at RHIC performed such a
measurement at forward rapidity (1.4 < η < 2.4) over the range of transverse
momentum 1.8 < pT < 7.0 GeV/c in polarized p+p, p+Al, and p+Au collisions.
We observed a positive asymmetry for positively-charged hadrons in p↑+p
collisions and a significantly reduced asymmetry in p↑+A collisions. The
measured TSSAs favor an A-dependence consistent with A^-1/3. These results
provide new insights for the study of small-system collisions.