That's a really good question for your specific application.
If it were HID lighting, I would say one monster would yield you the most in a six square foot, footprint.
With CFL's and their loss of lumens over a short distance, I'm not sure if one plant would get enough usable light to yield more than two smaller plants.
Either way the plant/plants cannot get tall. I can't stress that enough when it comes to CFL's. You want a short plant with a large canopy. At the least, topping or FIM'ing would have to happen a few times. Search SCROG grows as well, it's a little advanced but would be perfect for your lighting. If the plant becomes too tall it's very difficult to get CFL lighting to the whole thing. Yield will be small.
With all that being said. I think one would be your best option. Two might become crowded in that space. This can lead to stress, mold, stunted growth, and so on. One short, bushy plant, with well placed CFL throughout its growth should do just fine in your space.
This is all my opinion. Good luck with your grow! Keep us updated