I have a 2x2x3 space and the 250w and 400w systems are basically the same price on HTGsupply... could I pull off a 400w in this space? It would definitely need to be aircooled, but would it still be too much for my small space? If it is, I guess I'll just go with the 250
3 feet tall? That is way to small of height for a 400watt HID. You might be able to get away with aircooling a 250watt HID.
For that space the 250w is well matched. The only reason to buy more light than you need is if you see the possibility down the road that you will have a larger space and want to expand your op. Otherwise you are just creating heat problems and driving up your electric bill.