forced to build grow tent please help

so i am building my veg room as we speak and i cant do it properly because the shape or the walls either that or im an idiot. The room is shaped like a barn so it is very difficult to seal the room and make it work. It is a spare room upstairs in my a frame house. So i have decided to build a grow tent out of pvc pipe so it will be easier to wrap with panda paper. i already wasted a quarter of the roll the other way.lol. so i have 7 foot ceilings to work with. I will be running 2 400 watt mh for veg with 20 plants under them. i will veg for about a month until they are about 2 feet tall. HOw big of a tent should i make so i still have room to move them around and water them. I will have a small inline fan exhaust in this tent to move the hot air out and just a small passive intake. Any help on what to do would be great thanks.