we tend to overdo it, here is McIntosh's MC830 speaker gauge advice. (Earlier manuals noted smaller gauge).
The Speaker Wire
If speaker wire is not already terminated, remove
½ inch (12.7mm) of insulation from the wire end
and twist the strands together.
For runs under 25 feet (7.6m), use at least 16AWG wire.
For runs under 50 feet (15.2m) use at least 14AWG, and
forlonger runs up to 100 feet (30.5m), use 12AWG.
12AWG, being the larger wire, can be used in all
the above cases if desired.
The above guidelines are for 8 Ohm connections.
When using 4 Ohm speaker connections, subtract 2 from the gauge.
For example, a minimum gauge for a 50 foot 4 Ohm run
would be 12AWG. For guide, see Figure 07.
Generally, thicker gauge wire is better than thinner
until it doesn't fit in the Binding Post's hole.