I don't really like answering these questions because I have to give sad news. In the meantime, don't do anything to your plants; they're already in flowering and won't get any bigger. Now, I don't know how many grows you've done, but before thinking about an LST, I would try to understand why they're so small. To achieve what you see in the photo, you need a photoperiod strain; with an auto, it would be impossible to reach the level of the photo you posted in just 20 days. I took 4 months to finish one that was simpler. You can find an LST with a mini guide here in the grow I'm doing now:
growdiaries.com/diaries/164482-grow-journal-by-unkleskunk/week/914261. Now you need to understand why they stay so small... In the meantime, I hope you're using Rhizotonic as a foliar spray; if you use too much in the soil, it compromises its properties, but when sprayed, it's one of the best rooting agents. Also, in my opinion, the pot is a bit too big; I use 6.5-liter pots for autos at most, but usually 3 liters with coco. With soil, I use bigger pots. As for the light, I didn't understand what light you're using, but I think the plants also have light problems. For watering, if you're giving 15 ml of water every 24 hours, it's impossible for the plant to grow; I don't think I've ever given that little. I've given you some help; now you have to evaluate it yourself. Good luck!