Ok.. here is a rough idea.. hard to get perspective with a camera and distance..
I try and keep them about 2 inches.. you could get them closer if you really wanted..
I have a 3 inch air fan that just blows air around on the side for the sake of it.. if I had a tight box with cooling I probably still couldn't get them much closer..
Note how all the lights are of varying heights, some sitting surrounded by flowers on all sides.. maximizing your cfl spread is the name of da game.. put them together the lights cross over and shae some lumens.. if you put two 3,000 lumen blubs side by side, your immediate lumen output almost doubles. But if you spread them out by 5 inches, it starts to decline lumen share exponentially.. BUT you can spread them out.. one thing a SMALL hps (like you would use in this space) can't do very good is get into corners and edges.. it likes to concentrate down and center.. so you can use CFL to your advantage this way in corners and edges some..
And no, you don't have to have light surrounding each.. more light is better.. it would be nice in a perfect world.. but as you can see from this picture, the tall sativa, she is not only in a corner, but has grown 'above' the lights.. she is going to mature and eat just wonderful though.. I'll surround her with lights the last two weeks (soon) just to finish her off.. but she will still produce in a darker corner up against the wall and above the lights if she has to.. notice the extra leaf on the bud.. they are trying to compensate for the low light by leafing up.. that said they are getting and will continue to get quite crystal laden.. she still has at least 4 weeks to go.. freakin sativa..
This is what she looks like out of the room.. you may note she has even taken a bit of the shape of the corner I have her squeezed in to.
You can't tell from the pics.. but in a few weeks more time she really is going to be quite a monster for grown more low light than the others in the corner.