I'm growing 4 plants indoors under a 400w MH. My plan was to grow the plants between 14" - 16" (that's BEFORE flowering). Some plants slow their veg growth significantly during flowering and others tend to stretch to almost double their pre-flower size while budding.
The more plants you have, the shorter they can be when you go to bud. The less plants you have, maybe you'll want them to be a little taller (space permitting), so you can harvest more buds!