This return plenum box is designed to be used with a 12"x24" filter grille. The size of the box is 12"x24"x5" deep with a 3/4" flange, and the 8" collar is 4" long. Overall dimensions to the outside of the 3/4" flange are 13 5/8"x 25 5/8". It's manufactured from 26 gauge galvanized metal. The 8" collar will be cut centered in the plenum box for you to install it.  If you need a different size or a different size collar, please check our eBay store. You can call or message us if you need a custom size that you don't see listed. We will make and ship it out to you quickly.