If you want it to stretch, like let's say mine did... add more darkness and she'll bring out her photoperiod side of her and stretch like those genetics do, remember, most of these do have the traits back breed within them somewhere. Some will react more then others, like an auto made with AK47 might really get long legged... where a Kush that tends to stay more squat it might not react but a few inches.. Mine was done because I happen to be doing 1 dang photoperiod grow this year and I'm in midst of it , so it is what it is and she's very tall with almost no production, kinda sad really... LOL>
For the most part auto's that show sex fast and have not accumulated much size to them tend to stay on the smaller side.. I actually FIM cut mine to get her to stop and start filling out at the 2 nodes she has.... hehehe.. luck man