If you don't mind small plants, the smaller the better. The smaller your room is, the cooler and dryer it will run. Just pick a max height of plant and add at least a 12-14" for space between the bulb and the plants (hopefully it will run cool and you won't need that much). Add any height for pots and the height of the reflector you're using. Try and evacuate the air parallel to the bulb (have the exhaust of the room inline with the bulb).

It's really hard to predict when a plant will stop growing tho, so good luck. Try and use clones.