It's not that you can't use a HPS in that space, but it will need a good sized fan to cool it. My attic is 10' long by 5' wide (usable space) with a peak of 5' and I have a 600w HPS in one half, 250w MH in the other. It has a 240cfm fan on a thermostat, which has to run pretty much constantly while the HPS is on to keep it below 27C in summer, and it doesn't get too hot over here, 25C or so.
It's perfectly workable, as long as you plan for it and it might be a bit cramped working in there, especially when you fill it with plants!