You can only change the speed of the fan if it the fan itself uses a multi-speed motor which is not likely for the type of fan you have. If it did it would either come with a speed controller or reference an optional one. The fan speed controllers you buy are designed to replace existing speed controllers for multi-speed motors. They are not for any old AC motor.
What you should use for control isn't a dimmer anyway but a humidistat that will turn the fan on or off based on the humidity level in the room.
You can get ones that fit in standard gang boxes.