Selecting a specific manufacturer and model of an airfilter depends on engine size along with size and shape of the airbox. If you follow the Long-Ez plans, the airfilter is circular. If you build an airbox and attach it directly to the carburetor, it will probably be a different shape and size.
I tried to upload a one page article that Ben Owen wrote for a past issue of Sport Aviation, that deals with sizing an airfilter. I kept getting an error and will send it directly to you. Another good source of information is the CSA link on this website. Download the spreadsheet which is a directory of articles in past issues. Open the spreadsheet and search on "airfilter". I don't know if the articles are available on line or not. You might have to find a CSA member near where you live to read the articles.
Hope this helps.
Steve Rothert
Long-Ez N49SR
Albuquerque, NM