Ok, I have a 400W HPS -- bought it for a 2x3x5.5 tent and I think it was perfect for that size. My plants outgrew that, so I transferred everything to a 3x3x7 tent. My plants were tall at that point, so the light was still fine, but I did feel that tent was too big for that light. I would have wanted a 600W for that for deeper canopy penetration and for more power around the edges of the tent.
Hope that helps!