anything hotter than a 250W would be bad. You can enhance that small space with some fluorescent side lighting in addition to a 250W. Unless you had a liquid cooled system like a cooltube, which can get expensive and hard to find. A little less light is better than too much heat. And you don't have the height for moving a lot of hot air. Even with a 250W you will need an exhaust fan blowing out of your grow area. You will want to have mylar surrounding that area close to the plants. Try to make a mylar enclosure around them, but make a way for good ventilation and heat exhaust too.