Hey I had a similar issue where I could only grow my plants very few inches (around 15 inches). So I can help out, since I suceeded in keeping the plant this size limit. You will notice as the plant grows it branches off in seperate nodes. Whenever you want the plant to stop growing, you simply pinch off the next set of leave (node) that is starting to grow. So you're pinching off the tip of the stem. there for it stops growing upward. The two top branches will become new main colas. But keep in mind. Even though you top the plnt, it will still grow twice its size once switched into flower. So if in veg it is 5 inchs, it will grow to atleast 10 inches. I hope this helped. If you have questions lemme know.