I do not do this proffesionally. Last furnace installed by the same people. I felt the 100K BTU furnace had a slightly undersized filter 16 X 20. I was able to use the orginal large ductwork and cut a slot so I could use the orginal 30 X 20 filter. My new furnace just installed yeaterday. I spoke with the installer requesting a larger filter, said OK. The actual installes began making a 16 X 20 slot, and after some small talk I agreed to go with it. I didn't feel like going through an argument period. He said the slot is 16 X 20 and a bigger filter does not do anything more. When I go to the store all standard filters are the same price. For me, a larger filer has less restriction, last longer, and you can go with a better more restrictive air filter, like a 3M type. Maybe I can figure out later how to remidy the situation. Whos right?
GS