You always want to have two inches of space between the root tips and the edge of your container. As long as you do that, you can pot up as often as you want. Larger containers means less watering and feedings, but it also means less room to grow plants. So you have to find a balance.
The first time you grow a new strain, your not going to know how fast those roots are developing, so if you can, error on the safe side and go with a larger container. However, with autos the limit seems to be around 7 to 10 gallons. The plants only get 1-2 feet, so anything beyond 10 gallons is over kill IMO. In fact, many do fine in 3 to 5 gallons containers. A lot also has to do with your particular setup (timers, feeding frequency, planting medium, etc).
What has always worked for me is starting in 1 gallon smart pots using a good potting mix formulated for seed germination (I use Miracle Gro Seed Starting Mix), then I make sure to transplant before the end of the third week. Any longer and the roots will start poking out the bottom and sides of the smart pots. It also helps to let the plants get a little dry but not wilting just before transplanting and water immediately afterwords.
