length x width x height gives you cubic area, or the volume of your room. fans are listed as CFM (cubic feet per minute) and that tells you how many cubic feet of air it can exchange per minute. However, no dinky 6inch inline fan for $25 (duct boosters) is going to actually pull what it says it will, they boost air that is already moving.
the short answer: medical state and home protection plan (not lethal, neighbors). My house stinks, but my neighbors all like me and if their walk needs shoveling or their car is stuck, or if it's trash day and I'm lurking in the shadows until they go to take it out and I run over to help, or you ask how retirement is, you ask how the dogs are, you ask how the kids are....or if they want my right testicle and first born child...you get it. The guy next door works for the NSA...we don't talk a lot but we're cool. Not being the stereotypical pot head is the best thing we can all do for cannabis! Tip your freaking mailman if you grow and say hi, offer them water, etc....they are federal employees.
I do, however, constantly consider investing in scrubbers just in case some passing hoodlum catches a whiff...or if I want to have a bonfire in spite of our outdoor burn ban in my city....just because it's legal and I follow my numbers doesn't mean a cop won't smash and grab everything if they are there for another reason, that's how they roll.
I've streamed some of the videos for you because I have home internet and I can give you some information...this is the American way, I suppose. Dedicated scrubbers, which are just large fans attached to carbon filters and ported out of the room seem the way to go. Some people have the filter, the fan, then the light, and the exhaust after that. That's going to pull hot stinky air out, throw it across the light, and expel it. This is where pressure comes in though. some say that you need twice the intake (4 inch fan, 8 inches of intake), perfect when most tents have one upper hole and two lower of the same size. You can do an active intake though, where you have a fan going in and a fan going out. Luckily, there are a lot of text based threads that can tell you more about scrubbing your air. From what I have seen though, pressure and its effects on the efficiency of your fan is a pretty hot button issue in scrubbing....I had a tent that looked like a balloon - or a soft top jeep with the A/C on - and it didn't seem to hurt the plants. The ventilation guys are hard core so I hope they don't rip my pseudo advice up too much. Good luck, they're engineers. Sure, you could go right to them, but they don't have people skills, I'm good at dealing with people (going for that office space joke here).
Happy growing!