Square footage is commonly referenced in pellet stove documentation and marketing materials. Unfortunately, it is is not the most reliable way to select the right pellet stove for your needs. Unlike standard furnaces and boilers, pellet stoves do not use ducts or radiators to spread heat throughout a building. Instead, they are a central heat source that use some (limited) radiant heat and air movement to heat the space around them. This means that the further away from the heat source, the cooler it is. The way to increase the temperature is to have a larger stove that produces higher BTUs.
Pellet stove manufacturers often list a stove in terms of how many square feet it will heat. However, this is a really hard way to judge whether a stove will heat your house sufficiently. All houses are different depending on their size, shape, number of floors, tightness of the building envelope, and other factors.
While pellet stoves will heat a home better than a cord wood stove (due to the increased efficiency, as well as the air-heat exchange and blower system), it is still a central heat source. If your house is older and breezy, you might need a larger (higher BTU) stove to get the heat where you need it than if that same house had all new windows and good insulation. Similarly, if you have a brand-new house that has a very tight envelope, but it is long and spread out, you might need a larger (higher BTU) stove just to push the heat to the outer reaches of your home.
Pushing heat around your home with fans and other strategies can help even out temperatures, but if it's going out the windows, doors, and every crack and crevice, you'll just need a bigger stove. The best rule when shopping for a pellet stove is to get one of the highest BTU stove that will fit your home. They all turn off (many with auto-start) and can ramp down to a very low heat output. Another alternative, especially if your house is very spread out, is to use a couple of smaller pellet stoves in different areas of your home, rather than one central one. Your local pellet stove dealer can be an invaluable resource--bring a sketch of the layout of your home to show them when you are shopping for a stove.