New furnance filter

Hello,

I just bought a house that has a Aprilaire/SpaceGard filter system at the furnance. The previous owners just stuffed a Honeywell 20x25x4 filer in there and it is obvious it isnt the right size. i just had my ducts cleaned so what to make sure that I get a correct filter (son has allergies). Aprilaire has a kit that comes with one of those collapsable filters that has a MERV of 13. They say that it is designed in such a way that it does a good job of preventing air restrictions vs other MERV 13 filters. What is the proper way to determine if a filter is going to put too much strain on a furnance and restrict air flow?


Thanks.