How to Properly Size Your Bathroom Exhaust Fan - When it comes to the proper bathroom fan for the job, one size does not fit all. In this video, we explain the how to determine what size fan you will need to exhaust your bathroom. Once you have determined the size fan that you will need, a complete list of HVI-Certified bathroom exhaust fans can be found in the HVI-Certified Products Directory. To ensure optimal performance, we encourage you to choose a fan that listed in the directory. Not listed = Not certified.