I agree with Will, 3gal should be enough since they mature so quickly.
My best looking auto was directly in the ground in full Sun and a streetlight at night about 30' away. The buds were nice and dense with great flavor.
Being bred from a cold weather ruderalis, watch for heat stress. I would recommend an extra thick layer of mulch to help avoid the roots getting too much heat.
After watching mine, I agree with @Tonyb about a weak feeding. Water, water, feed. I went with a full strength feed on my third feeding and noticed a little nutrient burn on the tips.
If you go with a nutrient rich soil, don't feed anything until the buds are already formed and growing. After that feed bloom nutrients. I like Foxfarms, but whatever you prefer would work.
Don't expect a large yield. They grow too fast. Good for an intermittent harvest and fun, but not a heavy producer.
It's your first grow, so watch her and learn. With enough time, she'll teach you what she needs.
I'm not here as much as I used to be, but if you tag me, I'll respond as soon as I'm able...