I've grown 18 inchers under 2 "24" inch T12s. The key was topping the plant while using low wattage (40w) CFLs and 12oz Red Plastic Cups. The undersized pots allowed the clones to get root bound at "8" vertical inches after a 2 week 24/0 veg. In the end, it wasn't worth it as I only got about 4 grams per plant. I'm sure there are more effecient ways of doing it, but that is how I did it, and that is how it turned out for me.