I had a similar problem on my 2nd grow earlier this year. I’m new to this as well but I made a huge mistake of starting my autos in a small cup and then transferred to final bag and it stunted them hardcore and didn’t grow for about 2 weeks. By that time it was too late and they went to flower and one was about 6 inches, one was about 12 inches and the third a little bigger. Just plant in their final pot 3 or 5 gal and research watering. My 3rd grow I planted in a 3 gal with FFOF and did 24hours of light, outdoors during day and moved to a LED in my non climate controlled garage overnight. Kinda a pain moving it every night but It worked out. I did a little LST and have six good size buds getting fat and ready. I followed the advice of a reddit thread. https://www.reddit.com/r/Autoflowers/comments/20b591 . Not sure if I’m allowed to post it but it helped me figure it out and get some confidence. Just don’t follow the nute part if your using a fertile soil. He used a light mix and therefore used AN nutes. They are a waste of money IMO because they weren’t necessary with a Auto and good soil. Look for the ballin on a budget thread for some good nute advise or look for the secret to organic success by the infamous ComfortablyNumb if you don’t want to mess with nutes. Just have fun with it and remember less is more (especially with water and nutes)!