You've got about 4.75 sq. ft. of space. That is perfect for a 250w. But, if you're going to keep something permanently, I'd consider getting a 150w and putting 4 2 foot T5s along the back and sides. You'd end up with a bit less lumens (about 23,000 instead of 25,000 for a 250w), but you'd have fewer heating problems, slightly lower energy costs, and have terrific under canopy light. I wouldn't go with a 400w...that's enough light for an area twice as big. It's nice to say, "Well, more light is better!" And, to an extent, it is. It's also more heat and ventilation issues, especially if you're thinking of cutting the space in half. Just my .02 there.

I gotta know...do you have a Billy cabinet? I'm starting a thread in grow cabs about those...thinking about getting one.