I have a 1000 watter and it covers a good 8 square feet but...BUT....those outer couple feet ARE getting alot less light...so I try to keep my plants in a 6 foot by 6 foot area.
My advice would be to get a 1000 w HPS.
Also I move my plants and spin them under the light...as to even out the light each plant gets and make sure every angle is covered by moving and spinning.:thumbsup: