I would say that the 12" model would be maxed at 30x30, you could pull off a 36x36 but the outter edges would not have enough lighting or enough par to grow anything.
I have the 24" illumina and I'm Planning on hanging it above a 36"x30"x20" i think there will be more then enough coverage on the light, and as for par I dont have exact readings, although im sure there will be enough power to grow just about anything.
As it sits right now i have the light hanging about 6" off the water and have it turned down to 30% and even still i think i may turn it down more cause, i still have some bleaching still happening, hanging on a RSM 130D, so a foot print of 24x15x20ish?
This light kicks butt!
