Fan Sizes for Swamp-Cooler\Metal-Exhaust fan

What measurement corresponds to the size of the fan as sold by retailers. Is it the diameter of the wings, or the diameter of the frame around the wings. Also are they safe to use with dimmers commonly used for ceiling fans? I'm replacing a burnt one.

So, answering my own query after finding out. Apparently the size is the diameter of the actual fan, frame\mounting excluded. Good (gujrati) brands can be dimmed but 5-speed regulators are recommended instead of dimmers. Some local made ones have two fixed speeds (different coils switched between using wires).