The Corsi/Rosenthal Box is based on a 20" x 20" box fan.  This is because the size perfectly matches with 20"x20" filters.
Some countries have no box fans. Round fans have been used to build the Corsi/Rosenthal Box.
20" Box Fan - very important that the On button/ Speed Control is on the outer edges of the fan - or on the front where air comes out.  You do not want it on the back where air comes in.  Lasko 20" Box 3-Speed Fan Walmart Home Depot Utilitech 20-in 3-Speed Indoor Box  Fan  - these  also  come  in  black. Lowe's
Safety note: the box fans have been tested and found to more than up to the task.  With that said, as an additional note, it might be good to get one with a fused plug, which the Lasko and Utilitech fans have for sure.