To get the most efficiency out of a wind turbine that you are testing indoors with a fan, the turbine blades should have about the same length as the radius of the fan. Any portion of the turbine blade that extends beyond the fan blade length will not contribute to turning the generator, but it will create drag through the air and prevent the turbine from turning faster.

For a standard box fan, the blade radius is about 9.5 inches, or 24 cm.