Exhaust fans for your bathroom are one of the most beneficial tools that you could use in your bathroom, and some local building codes require you to have one even if there is a window present in the bathroom. An exhaust fan in the bathroom will come with a long list of pros and not a lot of cons, it will definitely remove the hot and moist air from your bathroom and on top of that it will keep mold and mildew at bay, on top of those functions it will keep your bathroom smelling fresh and you won’t need as much effort to keep it clean and tidy.
However, not all bathroom exhaust fans are created equally, and some are better designed to perform in larger rooms and the others in smaller rooms. While you can get an exhaust fan with high-power for your small bathroom it will be too noisy for the ambient and it will cause unneeded circulation which isn’t good. So you will need to find the perfect size for your bathroom, and when I say size this doesn’t mean the size of the device, but they are rated for the amount of air they can move in cubic feet per minute, and standard fan sizing applies to all bathrooms that are equal or smaller to 100 square feet of size, and generally I can say that you need at least 1 cubic feet per minute circulation for a one square foot area. Going with this math a bathroom that is 5 feet wide and 11 feet long results in 55 square feet area, and this means that your exhaust fan should be rated at least 55 cubic feet per minute or CFM circulation per minute.
However, there are still some things you should consider before diving in, for example going with the 55 square feet room, I would recommend with going a little higher than needed for example a 60 CFM circulation fan for better results, and also you should know that the minimum allowed fan size is rated at 50 CFM.
If you have large bathrooms you need to make different calculations, however, meaning that you will need to use different formulas for better results, and as a rule of thumb for a 100 square feet bathroom, you will need at least 50 CFM for a bathtub, 50 CFM for a shower, 50 CFM for a toilet and 100 CFM for a jetted tub, meaning that if you have a 100 square feet room and have all of the above you will need to have an exhaust fan of 200 CFM.
Other than that you will need to take the height of the ceiling into consideration, so if you have a higher ceiling you have to invest in a larger bathroom exhaust fan, and in some cases, you may need to install more than one exhaust fan.