DIY carbon filter, will this work for me?

So im starting a grow in a week and am planning on making my own carbon filter to combat the smell. My setup is a 3' deep*4' wider*8' tall. Ill be running a 400watt hps/hid. Its going to be 4 plants of bag seed. This closet connects directly to my attic via and this is where the air will be pumped. What Im wondering is how big/powerful will this have to be to keep the smell out? i also want to use this as my main source of ventilation to keep the temps down. I've looked at all the diy carbon scrubbers but i just want to know how big should i go, is there such a thing as to big? any help would be greatly appreciated

You can not go too big. You have a small grow and could easily clean that room with a 4 x 16 filter (or even smaller, depending on strain). There are many good DIY examples for you to pick the one that is right for you.

Best wishes.