You're going to want to make it as tall as possible, in my opinion. Sativas get really tall.... and making a high box gives you lots of wiggle room for you light heights. 6 feet is not enough as 2-3ft can be taken up by your hydro setup and the lights. And don't forget you need a space between the lights and the plants. 8 is great, in anything less than 7 I'd consider scrogging. Hell, the taller the better.

As far as your room size and money not being an issue.. I need specifics... How big is the room and how much are you really ready to spend? 3, 5, 10?