Yes, these are available in all standard sizes. But if you have an unusual dimension opening we can also accommodate you if you will allow us a couple of weeks build time. By the way, a good choice for filtration material is the type shown in the pictures which is available on a roll and cut with a scissors.
Most homeowners don't need a return air grille with a built in filter since they have an easily replaceable filter located next to the furnace. On the other hand, some people have a furnace in a creepy crawl space and they would rather avoid going down there to change a filter. Some people have a unit in the attic that is also inconvenient. These filter units will be a good solution for both these instances.