Calculate the volume of your bathroom:
Height of the room x length of room x width of the room = total volume.
ie. 8' high x 10' long x 6' wide room = 480 cubic feet.
Now divide the cubic feet by 7.5
480 divided by 7.5 = 64 cfm fan size. Or select the closest available fan size, when in between sizes, always select the higher CFM fan. Meaning, if there is a 50 CFM fan and a 75 CFM fan, select the 75 CFM fan.
There you go!