there is a formula for fan sizes, i don't know it offhand. A six inch for a 8x12 room will not be enough... its always better to have more air than you think you need, especially if you think your going to use one fan and have passive intakes. I used a 8inch H.O. Can Fan and it was not enough for my 11x11 room, once it was hooked up to a scrubber.
your fan needs to be able to clear your within 5 minutes of continuous operation as a rule. you'll need insulated ducting, a filter, and connections for your fan to scrubber. I aslo bought a big fat can ( you never know when your op might get bigger).