It’s important to have a filter that at least covers the entire area of the fan. A HEPA filter is most effective when a high air pressure can build up behind it. This forces all the air through the HEPA filter, removing the particulate air pollution. Air will always take the easiest route for it when passing through your purifier. If you have a HEPA filter that’s smaller than the fan, the large gaps between the fan and HEPA will be the easiest routes for the air to flow. This means a high air pressure won’t be able to build up behind the filter, and your purifier’s effectiveness will be drastically reduced.