Do you know what code governs your project?
Internationsl Residential Code 2003, Section R303.3 Light, Ventilation
& Heating, Bathrooms - says: Bathrooms, water closet comparments and
other similar rooms shall be provided with aggregate glazing area in
windows of not less than 3 square feet, one half of which must be
operable.
Exception: The glazed areas shall not be required where artificial
light and a mechanical system are provided. The minimum ventilation
rates shall be 50 cfm for intermittent ventilation. Ventilation air
shall be exhausted directly to the outside.

TB