Your grow box is 1 inch shy of being 7 square ft. So for vegging(3000 lumens min) you will need 21,000 lumens a 250 watt MH(20,000 lumens) is too small and a 400 watt MH(36,000 lumens) give you 5140 lumens per ft:aok:. So IMO the 400 watt MH would be the best for vegging. For flowering (5000 lumens min) you need 35,000 lumensa 250 watt HPS(27,500 lumens) is to small and a 400 watt HPS(50,000 lumens) will give you 7,140 lumens per ft this IMO would be best. You will have to have plenty of air movement thru the grow box to keep the heat from building and cool tubes would be the best IMO to help reduce the heat from the light bulbs. 1 fan for the lights cooling(cool tube) and 1 for air exchange. You total area is 36 cubic feet so a fan with a cfm of 108 or better will work great for air exchange and a fan with 150 cfm or better should keep the bulb good and cool.
:48: Hope that helps