I used to grow 1 Sativa and 1 Indica in a 18 inch by 24 inch space (And fit in extra, clones, veg plants etc).
Other people might want to grow one plant larger around then that..
If you want about a QP or more (if you actually know a bit of what you are doing) every two months, 24 inches by 24 inches and 300 watt's of light (short and in that space CFL works almost as dandy as HID ((IN some instances better))and produces fine too) can do it if you have yer sheet together.
LOT'S of variables, experience etc.. but I would start small, learn (assuming you don't have it yet) how to make a few small plants PRODUCE and then work your way up.
That's just me.. but it might add to your collective thoughts..
Peace~!