This is measured in cubic feet per minute (CFM) and the higher the rating is, the more air that can be shifted.
A more powerful fan will be able to remove the moisture faster, and a larger room can hold more moisture. So, if you have a larger bathroom a more powerful extractor fan may be needed.
An average size bathroom would typically have an extractor fan with a rating of at least 80 CFM.
If you need a more accurate calculation, measure the volume of your bathroom (length x width x height) and multiply that figure by an air change rate of 15 to 20 times per hour.
This will give you a result of the CFM of air that needs to be displaced each hour to find an extractor fan that closely matches your requirements.