Check the cabinet, not just the old label
Filters may have a nominal size, the rounded shopping size, and an actual size, the real measurements. For example, Filtrete lists a nominal 16-by-20-by-2-inch filter with actual dimensions of 15.5 by 19.5 by 1.75 inches. That example does not establish your cabinet's required fit. Filtrete product dimensions.
Deep filter cabinets can be especially confusing. Filtrete's refillable-filter instructions warn that nominal size can be labeled differently across brands and systems. Filtrete fit guidance.
Use the owner manual's filter-access instructions. If access is safe and intended for the owner, photograph the cabinet label, the old filter's full label, and the replacement's actual measurements. Note whether the problem is length, width, depth, or the way the frame meets the track. Do not reach into the furnace, cut the filter, bend a track, or leave the access cover partly open to make it work.
Get one reliable replacement record
Ask the service provider or cabinet manufacturer to identify the cabinet model, compatible media part number, actual dimensions, and acceptable filter type. If the former filter fitted poorly too, its label is not a reliable starting point.
Size and filtering strength are separate choices. A filter that fits does not establish that its resistance to airflow suits the equipment. Ask for both checks if moving to a different filter type. Do not stack thin filters to fill a deep slot or run heating without the required filter while shopping for a replacement.
Once the correct fit is verified, save the cabinet model, filter part number, and a photo in your home records. Your next purchase should start from that record, so this becomes a quick care task instead of another guessing game.