The air filter dimension is no accident. Manufacturers create filters this size for several reasons:Structural integrity: Larger filters have less structural stability due to their construction. Most frames are made from cardboard, which can bend or tear with enough air pressure. A filter is small enough to stay intact.Adequate air filtration: Filters for this size are large enough to provide proper filtration over a 30 to 90-day period before requiring a replacement.Compatible with certain systems: air filters are compatible with certain furnace and air conditioner brands and models.That is why air filter size matters. If you want optimal performance, you need to match the right filter with your furnace or air conditioner.