Air Filter size for (new to me) home HVAC
I do not believe that that large hole shown in the next to the bottom picture is for a filter. If you look to the left side of the picture that grey wavy item is probably the filter. To access the filter you will probably have to remove the bottom furnace door. You could also have a filter mounted in the bottom of the furnace.  I have no idea what that access hole is for but it probably is not for a filter or electronic air cleaner. I am sure glad the person that installed that return drop duct never worked for me. It may be a good idea to have a reputable HVAC company inspect the whole furnace and duct installation and correct any deficiencies. my 2 cents