Suggested Flowering Room Size

I am wanting to stick to the legal limit and grow six plants as large as I can. I'll be growing in soil, organically, with eventually 25 gal pots and CO2. I will have four 1000 watt HPS lamps and have the capability of using a space of 11' x 21' with 8' ceilings. The only thing is I believe that the amount of light would be spread too thin over an area that large. Any suggestions on a good room size to grow these six plants in? I keep seeing people say that ideal lumens are anywhere between 3,000 to 5,000 per square foot and then others say 10,000. And yet others say pay attention to PARS and a few other measurements of light. Anyone have a solid answer?

This will be my first grow and I am just trying to do the best I can from the get go. Thank you guys for any help. I've cruised this site for a long time doing a bunch of research and besides a little hatin here and there, pretty good tool to use.
 
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