4x 8 sounds great. I have a 2.5 x 7 where on one end is a 2.5x2.5 flower area perfect for 4 good sized plants. On the other end of the room I put a 2 ft deep shelf about 3 ft high and another 3 feet above that shelf then velcro and black plastic over the front and that's the veg/clone area with storage above and below it. Hope this gives you an idea. This small space produces 4-8 oz every 2 months. Perfect personal grow. 400w hps flower 4 26w cfl veg. veg room vented into main room, main room vented into attic.
I would guess you could do 9 good sized plants nice in there... 4x4 flower area. 2x4 veg with 4ft t5s leaving 2 ft for fans and getting in there too work. I also suggest leaving a half foot around the edge of you flower room, when plants are young it gives the light room to blanket them and gives them room to expand as they grow. They will fill that space that's why I say 9 plants not 12. Unless you plan on growing small plants but that would be silly right.
