14ga is kind of a standard for the average (not small) pike for track power. Big pikes use 10 and 12 sometimes 14 based on the run. 18 or 20 is only used on short 12" to 16" drops to buss when a buss is used.
Yes, folks do run smaller. "I can get away with....." and "That is good enough" has never an approach of mine.
Do not forget to use a variety of wire colors to help service your layout.
It can be a snake pit without some order.