The Coway Air Mega 200 covers 1748 cubic feet of space per their web site. If your ceilings are 8 feet tall, that translates to about 200 square feet (less if you have tall ceilings.) It is rated for a medium-sized room. This is the calculation to use if you are seeking maximum filtering capacity.
Ideally air exchange for the best results should be 3-4X per hour.