Air movement is essentially determined by a delicate balance between two primary factors: motor size and blade pitch. The size of the motor generally reflects its overall strength. There are three sizes that have proven to be effective and they are basically small (153mm), medium (172mm), and large (188mm or k55/xlp2000). The strength of the motor and the degree of the blade pitch determines how efficiently the fan will move air. The blade pitch refers to the angle of the blade relative to the fan. As the motor increases in strength, it is able to push a blade with a greater pitch or angle. Generally, the greater the pitch, the greater the air movement. But be careful! These two factors must increase in proportion to one another or else there could be adverse effects on the fan motor.