You can do much bigger than that in your attic, but for two plants I would get about 8 23 W CFL bulbs (or the equivalent) for vegging and a 250 W HPS digital ballast and cool tube for budding. That way you can do a SOG or SCROG and harvest twice as often. Make sure the veg bulbs are 5500K or 6000K color. Both the blue ones and the more common 2700K red ones will work, but the blue is better for vegging. I would think you would not have to vent to the outdoors if you keep it that small, but attics get darn cold in winter and darn hot in summer. You might need a heater and an AC unit. On those twisty bulbs in a globe, the globe cuts down the amount of light that is emitted, so regular CFL's are better for the same wattage.