it depends on how tall you want the box and/or plants to be, also dementions of the closet including doors and if you want to make two seperate boxes or just one divided, just one is cheaper but heat will be a slight issue, i have space for 5 and mines 3.5'wx2.5dx(about)3.5't, after this grow is done i'm going to divide it to have 3 in flower and do a SoG rotation and harvest about every 3 weeks, this is my first one in a couple of years and i'm tryin to regain my "weed legs" so to speakFuckn eh..g reat idea. would u know what size rooms i should divide it into? tryn to grow 4 maybe 5 plants. 2 will be autos 2 or 3 regs. i just have the one light.. but i have no problem running a few cfls for vegging side