For a DC layout, 18AWG or 20AWG is plenty for track wiring because you are only running one or two locomotives at a time on any given section of track.. Lights can be on 22AWG.
If you are planning on converting this layout to DCC later on, then you could consider using 14AWG wired up as a bus system for the track with 22AWG for track feeders to the bus. 22AWG will still be OK for lights on that size layout.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Wire can never be "too thick," but 14-gauge is pretty hard to work with. For track feeders and general structure lighiting, I use #22.
It takes an iron man to play with a toy iron horse.
I bought 14 gauage wire from HD to use on sidings and lights on my 12x7 layout w/ DC. Is 14 gauge too thck? If, so what gauge to use? I thought to have different color insulated wire so I differntiate between the wires.
TIA~
~Lee