Hi Farmer..
You can grow the plants as big as you want..
The biggest factor is NUMBER OF PLANTS... if you only have 4 plants under a 1000 watt HPS then you can wait until they are 12-24 inches tall before you flower..
If you have 10-14 plants under a 1000 watt HPS you will want to flower around 8-16 inches tall...
It also depends on hw close you can get your light to your plants. Personally, I focus a lot of attention on exhaust and ventilation and make sure that I can draw the heat away from my lights.. This way, the lights are always 12 inches or LESS from the foilage..
every 12 inchs the lights (HID) loose have their lumens (growing power).SUPER BIG think heavy bud HAPPEN when lights are close to the PLANTS!!
If your plants do get really big, you can always add side lighting.. Sidelighting helps tremendously. I saw a single plant that had a 1000 watt HPS bulb overhead with 14 - 100 watt ENVIROLIGHTS on the sides.,... for a total of 2400 watts on a single plant.. The plant yielded 3.5 LBS....
freaky HUH!!
iloveyou