3' x 4', roughly, for a 600-watt light. For a 4' x 4' space, one 1000-watt light is good, or two 600-watt lights. For a 2' x 3' space or a 3' x 3' space, a 400-watt light works. For a 2' x 4' space, a 400 watt light works okay, a 600-watt light is better (if you can handle the heat), or a 400-watt light plus some accent CFLs at the far edges works well.

I think your plan to go with a 400-watt light makes the most sense for your (2' x 3') area. Better to use a 400-watt light and find out you need a few cheap accent CFLs to go with it than to use a bigger light and find out you can't use it at all because of the heat.