Lets say you get one rated for 200 sq ft or 400 sq ft but use it in an 800 sq ft room.
Will the air purifier still cycle all the air in the room, just do it slower than it says it will, or will it not have the power to mobilize and clean the air outside of the 200 or 400 sq ft area around the purifier?
I use a HEPA Enviracaire/Honeywell air cleaners, and I think the amount of coverage is for the rate of exchange. For example, 400 sqft rate exchange is done in 4 hours. I’m making these numbers up. But what I would do is contact the company’s customer service by email or phone and ask them. I have purchased these units for the size of the room it will be used in. Even if in my example it took 8 hours for the rate of air exchange instead of 4 hours, that might not be enough coverage in the amount of time to make sure the air is of higher quality. Depending on the source of the allergen taking double the time might not be useful. Again, I’d ask the company who makes the unit.
The other thing to keep in mind, is most of those units have a speed control on them, so that matters also.