Herb can grow from anything to a couple of feet tall, up to 6 foot. It just depends on the strain, when you look at the seeds on the net (www.everyonedoesit.com is a good place to buy - cheap - and they also do bloody loads of seeds) they will tell you the percentage of THC, how high they grow, how easy/hard it is etc. Lowryders only grow to a maximum of 2 foot, and thats in rare circumstances. Ive got a diary in this section, havent updated in a week or so but shes not much taller than the last set of pics show her. With 8 lows, you can expect a harvest of around 2-3 ounces easy.
And that sounds like a class idea (moving the lights as they grow). See if you can get a light on the side of them while theyre budding aswell, the buds will form more compact and give a slightly bigger yield.