Why is mostly square, radiator cooling fan can make a rectangle?

We see DC DC fan, such as computer CPU cooling fan, box fan, cooling fan, fridge converter cooling fans, switches, cooling fan, etc, electronics, electrical appliances, industrial equipment use DC DC fan, mostly are square, the small part is circular or without outer frame, can make it long? Cooling fan for our common axial flow fan, fan follow suit to a 90 - degree rotating direction, because each piece is the same as the length of the blade, so, turn up, a few leaves are formed a circle, outside the framework can only be square or round, of course, if you must make a rectangle, can only outside on both sides of the frame to lengthen, but the blades still remains the same, the same as the original square DC fan or cooling effect. , of course, if the customer must be rectangular, it happened that way of heat, can also be, as we have a large fan on the door at the mall, the length of the similar to the door, but is very narrow, this belongs to the cross flow fan, blade is long cylindrical blades rotating direction is the wind direction of the wind.