dude you need to read.. but i say no smaller then 10 inches and during flowering the plant will grow 2-3 times bigger. i started to flower my sativa plant at 13 inches 2 weeks ago and it is now 23 inches
They will keep growing for at least 4 weeks after you flip the switch. So the time to flower is dependent on how much space you have, and the type of grow you're running. So the answer is different for everyone.
When your plant reaches half as high as u want to grow, usually doubles in size. Plan out your grow space and that determines when you should flower.