How much space do you have to work with once you light is as high as you can put it? From what I gather a plant can double to triple size during flowering. So if you have 3 ft to work with flower at a foot and a half. I'm glad to hear your plants are filling out. I'll have pics of mine up later today, you should check them out.
http://boards.cannabis.com/showthread.php?t=97106