That's a pretty small area, but it should be plenty big enough for two plants. In a small area heat from your light might pose a problem. I wouldn't use more than a 400 watt lamp. However, without proper ventilation and air circulation, even a 250 watt lamp could bring about heat issues. If you think that your ventilation is so-so, then try a 250 watt light. Otherwise use a 400 watt light.
Good luck.
The Hemp Goddess made a very nice cool tube for her pair of 150 watt lamps. I believe you can find a link to her thread in the DYI section of this site.