a/c filter
 

what size ac filter do i use for a trane xr 80

Depends on the size, location of the filter (internal/external) and many other things. The XR80 is a "line" of equipment, not a specific furnace.

The existing filter should have a size on it, assuming it's a standard throwaway filter. If not, then just measure the filter, head over to your favorite filter-buying location, and pick up a few.

If you don't have an existing filter, then you'll have to determine where (in the system) the filter is placed. Many systems have "filter grills", which are located directly in the living space. Others have filters in the bottom/end of the furnace, and still others have filters in racks mounted directly in the duct system.

Also determine your efficiency, standard throwaway, merv8 ,merv11 or higher