This is how subcool figures HID lighting (he said this is for all HID lighting, it's not specific to MH or HPS).
1000 Watt Hoods…………6x6
600 Watt Hoods………….5x5
400 Watt Hoods………….4x4
250 Watt Hoods………….3x3
You can find it in his Canopy Management thread in Subcool's Organics.
I would think this has a lot to do with it as well as the overall environment of course.
The more light footprint you have the more plants you can fit in there. You also have to figure about the amount of lumens being distributed. A mean a 250watt could like a 10x10 room to you eyes to be able to see but you really have to figure out if you plants have enough light to grow in.
I think that chart is pretty much right on. I do use a 600watt in a 4x4 though and I figure it may give them a little more than they need but I don't think I would really get more bud off them but just to be on the safe side with everything. Better be better off than lacking right?
I also figure with it this way I would be able to grow my plants bigger and this brings us to the next thing would be plant size. You can get by with using less lumes on plants that are small rather than big. If you have a smaller lamp and big plants I am not sure that your bottom branches will be getting the light that they would need to fully develop. This can be gotten around by either growing a lot of small plants or perhaps lollipopping the plants. I don't know and am still experimenting.
Im my situation I am growing my plants bigger under the 600watt in a 4x4 area. I will lollipop them too so that there is maybe 6 to 8 cloas with one being the biggest. Say if you had a 400watter you might not be able to get away with this aswell. You would be better off either getting a screen and screen out one or two plants or doing a sea or green (by that I mean just having a lot of small plants)
If you have a lot of small plants of a really good canopy you can get the light right close to the tops (get it close too cuz the 400watters run pretty cool). That is a whole nother issue is the heat givin out by these lights, and with more and more people using those grow tents, the HUTS GET HOT!!! I could go on for a while about that too.
So basically just be as effeciant as you can with the light you are given. Smaller lights = get them close to the canopy and use all the lights at the point where they are the brightest.
Bigger lights= may give you better penatration and better ability to grow bigger plants but you have other issues that go along with that such as heat and electricity usage.
I also saw a chart on RIU going around that shows how many lumens your light looses as the plants get further away. It was saying at like 4 feet is is like 1/32 of the bightness. Im not sure if it was that exactly but it was a substantial loss, sometimes if you just bring the light down by 1/2 the space you can double your yeild. A lot of people have the lights to far away. Be careful not to burn but if you spend the time with your room it will be no problem. Just get a thermometer and hang it at the canopy and watch it for an hour. If it gets to 120 then you need to back off your light a lot. If it is 98 you can tinker with the light and rase it a couple inches put a fan on the light and then check it again. It just takes a little effort and time in your room and it will be good to go.