Bob,
#12/3 SJO (water resistant) is a standard "stinger" cable in use for lighting work, but technically is rated only at 9.3 amps. In practical use it's common to load the circuit until the 15 or 20 amp breaker blows, so anything less that
#12 wire is inadvisable. Furthermore you can expect voltage drop with heavy loads at longer distances, more so with thinner wire. Best practice is to use heaviest wire, calculate load and occasionally feel the cable and connectors for warmth!
JS