Depending on the ballast it should be able to run different length bulbs. With my VHO ballast I can't run more than 220 watts, no bulbs longer than 4ft, and no more than 2 bulbs. I could use it for 2 ft bulbs, but right now I have 2 3ft bulbs. Hope that helps. Ballast is ARO. Just do a little research on the ballast.
-Kris