@Sue Sorry to come late to the party... Just wanted to to say that even with perfectly working exhaust fan, you'll likely still see a bit of moisture while you're taking a shower.
Make sure you run the fan for about 20 minutes (or until all the moisture is gone) after you're done.
If you had a fan strong enough to immediately suck out all the moisture from a shower, it wouldn't be very comfortable to be in the room.
Check out this answer for more information.
A quick formula to estimate fan size is:
CFM rating of fan = room_length * room_width * room_height * air_changes_per_hour * 0.0166667
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