What size aluminum ser do I need to feed 100 amp subpanel? Ten feet from main panel.
With surface mounted panels in what presumably is an unfinished space, there is 0 reason to run an exposed SER cable here. Instead, I would run a conduit between the two panels (2" EMT will handle anything you want to do here), and then simply run 3 XHHW-2 conductors (hot, hot, neutral) in the conduit between the two panels, using the conduit as a grounding path.
As to inner conductor sizing...
I would use 1AWG Al wires here; spending the extra for copper is a waste, and you can't get away with 2AWG for this because you'd exceed the 75°C termination limit that is normal for breakers and loadcenters if you did. This limitation is a consequence of NEC 110.14(C)(1), by the way.
TORQUE ALL LUGS TO SPEC
One essential thing with this job, no matter who does it, is that all lugs will need to be tightened to the torque specified on the corresponding labels using an inch-pound torque-wrench; this is required by 2017 NEC 110.14(D), and is a good idea anyway, lest your electrical system go the way of Greg Biffle's infamous lugnuts.