Remember, you are filtering out VOC's from the exhausted air, hopefuly not dust/dirt, so it's getting the maximum air flow through the filter that is most important. The largest 8" Terrabloom carbon filter with attached 8" EC back pressure compensated fan has worked exceedingly well for me, exhausting both an 8X4 and a connected 5X5 tent. It creates enough negative pressure to suck chrome off a trailer hitch:
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I can't afford to have anything taking up or impeding the headroom _inside_ my tents:
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And before the helpful crowd gets geared up to inform me I am over-feeding, over-watering these plants, the temp inside this tent when I took this picture was 48° and the temp inside the basement was 40°. And this is the sativa side of a reverse engineered Northern Lights, and if you're familiar with Northern Lights, you'll understand how atypical this is.
I've got to have the tent headroom inside and the smell has never, ever been a problem.