Bob,
Wire usage is determined by the current the load to be powered will require, the length of the run, and the voltage needed at the load.
Higher guage numbers mean smaller current carrying capacity and higher voltage drop per foot.
Normaly you can do a trade off, however you have to meet the required voltage and current requirements at the end of the run plus enough of a safety margin to prevent melted wires, insulation, and a FIRE.
Here is a nice place for information:
http://www.powerstream.com/Wire_Size.htm
If the item you are going to power requires under 0.9 amps (n scale loco for example) then a AWG 22 feeder at the rated voltage would work. However you must take the length of the run into account to make certain that you are fine.
Now what most folks would do to make certain that everything was going to work smoothly would be to run a large guage bus under the track line and run smaller guage feeders off of the bus. This serves several purposes, the only one I'm going to mention is that it is makes a good solid set of eletrical connections.
Remember to perform the old standby short test (quarter across the rails, etc...) to make certain your power system breakers trip without causing any damage.
It always a good idea to be conservative in the area of electrical wiring.
DCC also requires a better set of connections because the command system is always talking to the decoders. As I understand it long DCC power runs should also be properly terminated because of this and a related decoder destruction possibility.