There are two considerations, voltage drop and heat buildup.
	The smaller the wire is, the higher the resistance is.  When
	the resistance is higher, the wire heats up more, and there is
	more voltage drop in the wiring.  The former is why you need
	higher-temperature insulation and/or bigger wires for use in
	conduit; the latter is why you should use larger wire for long
	runs.

	Neither effect is very significant over very short distances.
	There are some very specific exceptions, where use of smaller
	wire is allowed.  The obvious one is the line cord on most
	lamps.  Don't try this unless you're certain that your use fits
	one of those exceptions; you can never go wrong by using larger
	wire.