HVAC filter sizing?

My house has central air and is ~3200 sqft including the finished basement. It has a single 16x25x1" HVAC system filter and I have a strong suspicion that is is very undersized. Even with a fresh filter, it seems like the fan unloads pretty well with the filter in place, and even if you just slightly slide the filter out so that air can get around it, the fan loads down and flow to the vents seems to increase substantially.

Any experts here? How do you determine what size filter should be used?

 

I’m not a expert, but I believe you would need to have your static pressure measured, and put into a load calculator type app. Also depending on your area and your equipment, the unit is set at a certain CFM per ton.
I have a 1400sqft house near Satan’s anus, and I’ve got a 4ton condenser feeding a 3.5 ton indoor unit. My filter is a 20x25x2 that gets changed every 30 days regardless