How do I know which fan size to buy?

Three Fan Design – 2803-V
Family or great room ( 200+ sq ft) , cathedral ceilings greater than 9′, larger 30 – 36 inch wide Direct Vent Fireplace
Four Fan Design – 3804-V
Large family or great room ( 200+ sq ft) , cathedral ceilings greater than 10′, larger 42 – 50+ inch wide Direct Vent Fireplace
Variable Speed (-V)
Variable Speed allows you to adjust the speed to achieve the minimum sound level and still keep the mantle from becoming hot. It can be adjusted to run slightly faster and and significantly slower than the fixed speed.