First of all, I don't think you should use aluminum wire if you are planning on soldering it to anything. I believe (maybe someone can confirm) that it is difficult or impossible to solder, and that there may be corrosion problems if it comes into contact with copper.
As to what size, things to consider are DC or DCC (the latter typically needs heavier wiring) and the length of the wires (the longer the wire the heavier it may need to be.)
As a starting point, let's say use No 16 AWG (12-14 if DCC) for bus wiring, and No. 20-22 for feeders - the bus being the long wires under the layout and the feeders being the short wires connecting the bus to the rails. Bigger is better, or at least won't hurt, except when soldering to the rails.
Sort of a tangent, but does anybody know what AWG a rail would be equivalent to? (Code 55, for example?)