It's critical to have a good ventilation system set up in your bathroom to keep humidity levels in check and to inhibit mold growth. The Panasonic Energy Star 80-CFM bathroom fan is one of the most energy-efficient fans you will find in the market. It's also pretty quiet, producing only .4 sones of noise, and extremely durable with a lifespan of 100,000 hours.
The grill measures 11-13/16 inches square and fits a 4-inch duct. It's a great, non-fussy yet effective fan for a reasonable price. It retails for $96 on Amazon, and has terrific ratings there as well.
If you have this fan, tell us about it!