I noticed my Merv 16 filter (and filter box) in front of the furnace blower is bigger than the blower opening. I've wondered if a duct transition should be added, but don't want to go to the trouble unless really necessary. Any thoughts on whether this is a problem? Probably will run through the filter quicker since the top of the filter is not getting use, but I could live with that as long as I didn't cause air flow problems from having such a high performance filter and then not even having it set up where the whole filter will permit air flow.
See the pic below (which is sideways...the furnace lays horizontal). The filter box is about 16x25 and I bet the furnace opening at the end is about 12x24.