A general rule is that a one-gallon pot will support a plant for one month of growth. With a 3-gallon pot you can count on 3 months plant growth. As few autos need that much time you can't really go wrong with something in the 2-3 gallon range.
Pot size does make a difference in size, but maybe not as much as one would think. I have experimented some with the subject of pot size vs. size/yield. With my usual strain, Blueberry Gum via G13 Labs, I can grow 36-40 inch plants that yield 25-35 grams cured in a 3/4 gallon berry carton that is 7 inches square. Using the same strain my yield increases 10-20% in 1 1/2 gallon pots and another 10-15% in 3 gallon pots.
About that gallon per month ratio - the only time I break out any pot over 3 gallons is when I'm experimenting with some sativas that take 12 weeks just to flower, or when setting a plant up to be a mother plant.
Good luck, BigSteve.