You can grow a lot of plants, you just have to keep them small. Or you can grow a few big ones. My space is that size and I grow 6 smallish (3ft) plants under a 400w hps. Set up your light and find out if you have a heat problem BEFORE you start growing. Keep in mind you WILL bring in air when you exhaust air - you won't create a vacuum. So where will that air be coming from? It won't help to exhaust 85 degree air if you bring in new 82 degree air. How cool can you keep the room outside the closet where the replacement air will come from? That's the key.