Why do I need an exhaust fan ?
Apart from local building regulations, it is important to ventilate well wet areas so as to avoid future humidity damage to walls and ceiling. Ventilation is also important in toilets to dissipate odurs produced.

Will any fan do ?
An exhaust fan needs to be sized according to the room size. This is quite logical as the fan moves air and the larger the room, the more air the fan needs to move. All exhaust fans should clearly display their air movement capacity to allow designers and consumers to make an informed decision on its suitability.
What capacity should the exhaust fan be ?
Generally 10 air changes per hour should suit most bathrooms, toliets and laundries. Work out the air volume in the room (height x width x length), then multiply by 10 and that will give you the capacity required of the fan.
Will the exhaust fan stop the mirror misting ?
Probably not ! The exhaust fan takes excess moisture out (but not all) and it is a good idea to let it run for a while after finishing a shower to clear the moisture. A strong fan can create an unpleasant draught and take most of the warm air out of the bathroom, creating an unpleasant environment.