How do I size an air purifier to my room?

Treat the manufacturer's headline square-footage claim with skepticism. Reviewers and r/AirPurifiers commenters consistently point out that big claimed coverage numbers are based on a single air change per hour, while real allergy or smoke relief needs roughly four or five. As a rule of thumb, divide the advertised coverage by four to five for genuine cleaning performance, which is why a unit rated for 1,000+ sq ft is often the right choice for a 250 sq ft bedroom.