The two plants I had in my room recently were taking up about 22"X36" of my grow space. If your reflector ends up looking like it will span 36-40" in any one direction you could go with 30"X40" and be right around the 8 square foot mark. You could have 6 decent sized plants, or four bigguns in that space as long as you have good ventilation. I say that because you'll notice, or may already have noticeed, when you water the plants the temp and humidity go up almost immediately and stay that way for a day or so. If you have good ventilation it minimizes this condition and keeps things from getting to a point where you run into trouble.
I kind of wish I had made my veg room longer so I could run 4' flouros but I didn't so...
Too bad you don't live where I do or I'd offer to come over and help ya build it. Construction is what I do and I love doing this type of stuff so I'd help anybody that wasn't a giant tool, haha. I've aleady made a little side money building a couple cabs for a couple kids that work at local dispensaries. I did a SCROG hydro cab, a regular free standing cab and a stealth cab made out of a nightstand so far.