i know from my first grow that they will just about double in height when they flower. i don't think it matters when you start to flower. sounds like you need to start now. my northern lights are just over 12 inches and i just started 12/12. they will be 2 or 3 feet when done. toping is good to keep it short and you can also take the lower water leaves off to promote branching. you don't want to trim or top when they start to flower you will lose bud sites. kind of a small space. what strain are you growing ? I tried some bagseed the first time that went from 16 inches to a little more than 4 feet. you might want to read up on scrog (screen of green). do a search. thats kind of what I did to keep the bag seed from making me raise the light away from the smaller strain. you can lay them down and tie it to stakes. do a search there's lots of good info and pics. hope this helps.