Okay, here we go me playing devils advocate. I have a 400w MH light with a conversion HPS bulb on now for flowering. My space is 7.8 ft hi, 4 ft wide, and aprox 4 ft deep. This is my flower room only. My space is in a former cold room, and I have a 6 inch vortex with a T'ed exhaust. working on the cabon filter. This room is long, and I have about 15 ft outside of that, for my veg area etc. all 4 ft wide, long and narrow. LOOK HERE to see how I set it up, Chiefa. Lots of info and different ways to conceal vents etc. I hope it helps you.
MY LITTLE GROW...or What not to do 101A
IF I had my druthers I would of bought a 600 watt hps with a Sun System Cool sun reflector. Here is another discussion by people that know what they are doing.
1000 watt question?
For your space, 1000 watts would be too big, but you will not go wrong with a 600 watt one. even a 400 watt one will be fine. And one is fine. In my opinion. In my opinion it is also worth while getting one that is convertable from MH to hps. I have heard differeing opinions on this, but that is my thought. Just hps works as well for veg and flower though.
Hope that helps you sweetie.
Peace.