You could easily grow two lowryder 2 plants in there. Anything else would be difficult. You'd have to spend a lot of time tying down every branch for SCROG. Your CFL's will take up at least 4" and your pot has to be at least 6" tall.

So if the lights are mounted sideways (which is less efficient), thats 4" +1" distance away from plants. That leaves you with 18". Now your grow container has to be at the very least 6" so that leaves you 12" for the plant. Its totally doable, but very high maintenance at first. Once you got everything tied down and a screen in place, you can relax a bit.

If you go with auto flowering strains, they'll grow fine in that space with minimal to no tying. You'll need really good cooling to run 3 42w bulbs in that space. 4 23w would work for both veg and flower. Just don't expect big dense buds because a box that small isn't exactly ideal for growing quality stuff....though the auto strains come out pretty damn nice.