FWIW I have virtually the same thing (made from the plans on the Plamondon webpage), mine is 2x4' and is good for 20-30 chicks but I am skeptical about putting more in it if the building/weather were cool enough that they'd continue wanting heat up into the 6-8 wk range.
With mine, it depends on how many chicks I have and what the temperature in the chicken bldg is, but I usually start out with two 100w bulbs in there, and work down from that as the chicks get older. After a few weeks I will only have one 60 or 100w bulb on in there. In colder conditions you might need more than just 100w bulbs, too. It is best to have an assortment and you can mess around with 'em til you get the effect you want. Ideally BEFORE the chicks arrive <g>
I leave mine set up for a month or so after turning off the light, and they do huddle under it at night, which I assume is what Plamondon is talking about with the "for pullets [...] but without electric lamps" thing. They also like to sit/play on it during the daytime.
(e.t.a. --- you for sure DO need the lightbulbs on for younger chicks. When Plamondon says "for pullets" he means "for chicks past the point of needing lots of heat but not yet fully mature")
The main thing with this kind of hover arrangement is that it does zero against drafts, so it needs to be in a really good draftproof building. In a sectioned-off part of a coop, you might need to put 2-4' high cardboard walls on at lest one side of the pen because some breeze will come through the adult chickens' popdoor. (If it is in a garage or other closed building, that would not be an issue).
The only problem I have had with my unit, in maybe six uses now, is that I had one chick burn his comb on the bulb at maybe 2 wks of age. (Because the thing is so low, there is really no way to keep the bulbs up out of the way of chicken heads, nor is there a safe way to have a guard on it). But, he got over it, and the problem has not recurred. To me that is an okay tradeoff for it being otherwise a brooding method that the chicks seem to really like and do well with. Oh, and it is kind of big and awkward to store. But, oh well
Pat