usualy plants double in height when you flower so it is a good rule of thumb to go by. If you know how much space you have vertically to use that to decide if it is going to get too big or not.
You want to flower 8 - 9 weeks , longer for sativa strains
They can double, or even triple in size. Just flower when your plants are half the height of your room. And if you are really worried about height, just tie them down Before you flower