I would recommenced using a 5 or 7 gallon pot. I used a 3 gallon and got root bound mid flower. I just finished a BB auto grow. I transplant from the dixie cup to the final pot at day 10 for mine. All my autos are in 5s now except my magnum. it is in a 7. I am also going from a Dixie cup to 1 gallon then to a 7 for this magnum I just started. If you would like to see my Big bud in a condensed form.
Check out the link below and the pot number 562, 584, 586, 603
It will give you an idea what to expect with your plant. Mine was in a 4x4 alone. it was about 40" tall and 42" wide
This was an easy plant to grow. My friends are all enjoying it now 
I use coco for my grows
I also use a 23w cfl light over the seedlings at about 2 fingers off the plant, till they get put in there final pots. has worked out well for me. I start every thing off in rapid rooters my self. Just how I learned. I have also topped every auto I have done with no issues.
Good luck with your grow. And this is just letting you know what works for me. There are a lot of right ways to start these off.