If the filter is too small
A filter that leaves gaps around its frame allows some return air to take the easier path around the filter media. That means dust can bypass the filter and move farther into the HVAC system.
The filter may also shift, rattle, or fail to stay seated correctly.
If the filter is too large
A filter that is too large may not slide fully into the slot. Forcing it can bend the cardboard frame or damage the filter media.
Even if it can be squeezed into place, a distorted filter may not seal properly and can become difficult to remove later.
Thickness matters too
A 20 × 25 × 1 and a 20 × 25 × 4 share the same face dimensions but are not interchangeable. The third number is part of the fit.
Use the same nominal thickness specified for the slot or media cabinet unless the equipment manufacturer says the cabinet accepts another size.
Nominal versus actual dimensions
Remember that a correctly sized filter often measures slightly smaller than the nominal size printed on its frame. That does not mean it is too small.
Match the nominal size printed on the existing filter first. Use actual dimensions as supporting information rather than inventing a replacement size from ruler measurements alone.