For a couple years I had heard that auto flower plants drive their tap root straight down and when it reached the bottom that signals the plant to start flowering. I thought, I've got to know if this is true or not. So I found a cheap, clear pot about 2 gallons in size that I fit into a slightly larger blue pot. I started the auto flower seed in a perlite/hempy solo cup and at day 3 transplanted it into the clear 2 gallon pot. Each morning I would take the clear pot out, hold it up over my head and look for roots on the bottom. On day 12 I had the first hint of roots having reached the bottom of the pot. So the plant at this point in time was 15 days old. Ten days later, with not sign of flowering starting I carefully up-potted the plant into a 5 gallon bucket. The dirt clod didn't cooperate fully and a couple chunks of dirt fell off. But all in all I was happy with the transplant. The strain was Dark Devil auto and the plant began showing the first signs of pistil formation on day 33......18 days after the roots hit the bottom of the 2 gallon pot. Did that experiment prove definitively that the plant starts flowering when the taproot hits the bottom? Nah, but it was enough for me personally to dismiss that claim as fact.
One thing I do know to be true about autos is that it's incredibly important to stress them as little as possible because as
@GreenGenez said autos have a tremendous amount of things to get done in a very short period of time. I personally feel that if you are really careful transplanting doesn't have to be all that stressful for a cannabis auto plant. With that in mind, and having had great success in the past following the teachings of
@Emilya I would feel comfortable stepping into a 1 gallon pot. Question is when do you go from the 1 gallon into the final size pot? Too soon and the soil may fall apart which could stress the plant.