Current codes requires exhaust fans in bathrooms. While this does not affect your's, a fan is always a good idea. Fans must exhaust outside. If the bathroom is on an outside wall, or has an open attic above, installation is fairly simple. If located towards the interior of the house, it becomes more difficult, but not impossible. As to sizing, measure the square footage, multiply by 1.5, that should equal the CFM of the fan.