Your fan should achieve eight air changes per hour. To figure out what size fan you need to achieve this, you need to measure the cubic feet of your bathroom.
You can get this number by miltiplying the length, by width, by height.
Next take the cubic feet and multiply that number by eight, for the eight air changes you need.
Exhaust fans are measured per hour so you now need to divide the number by sixty, sixty minutes in an hour.
This room is 500 cubic feet which means I need an exhaust fan with sixty-five cfms or higher.
Now you also want to take into consideration things like steam settings and jetted tubs when determining your cfms. These types of elements add moisture to the air and require more power to remove that moisture too.
A little tip about running your fan: start the fan before you start your shower and leave it running for at least twenty minutes after you're finished in the bathroom to be sure that all of the excess moisture has been removed.
I don't know about you, but I'm exhausted. I'm Mrs. Fixit and it's just that simple!